IN 


John  Swett 


X 


FIRST   BOOK  !>.j:-A/ 


NATURAL  PHILOSOPHY 


FOR   THE    USE   OF 


antj 


BY 


J.    A.    GILLET, 

PROFESSOR   OF   PHYSICS   IN  THE   NORMAL   COLLEGE   OF  THF.   CITY  OF   NEW   YORK, 


W.    J.    ROLFE, 

FORMERLY   HEAD   MASTER  OF  THE   HIGH   SCHOOL, 
CAMBRIDGE,   MASS. 


POTTER,    AINSWORTH,    &    CO., 

NEW   YORK   AND   CHICAGO. 
1884. 


• 

^•' 


COPYRIGHT,  1882, 

BY  J.    A     G'LLET   AND    W.   J.    ROLFE. 

EDU^ 


JFranWin 

RAND,    AVERY,    AND   COMPANY, 

BOSTON. 


PREFACE. 


THE  authors  have  endeavored  to  present  in  this 
little  book  a  brief,  simple,  and  accurate  statement 
of  those  facts  and  principles  of  natural  philosophy 
with  which  every  one  ought  to  be  familiar,  and 
which  may  at  the  same  time  serve  as  a  foundation 
for  a  more  extended  course,  in  case  the  student  has 
time  and  inclination  to  pursue  the  subject  further. 
They  have  thus  sought  to  make~  the  book  really  what 
it  claims  to  be  in  name,  —  a  first  book  in  natural 
philosophy. 

Great  pains  have  been  taken  in  the  selection  and 
arrangement  of  topics,  and  in  giving  due  prominence 
to  each.  In  order  to  make  the  book  sufficiently 
brief,  and  at  the  same  time  to  do  justice  to  the 
recent  remarkable  advancement  in  our  knowledge 
of  the  forces  of  Nature  and  in  their  practical  appli- 
cation, it  has  been  necessary  to  omit  certain  illustra- 
tive matter  which  some  teachers  will  be  likely  to 
miss.  We  believe,  however,  that  the  illustrations 
and  experiments  given  are  all  that  are  needed  to 
make  the  text  clear.  Others  can  of  course  be  added 
by  the  teacher,  according  to  the  condition  of  his 
pupils  and  the  supply  of  apparatus  at  his  disposal ; 


IV  PREFACE. 

and  they  will  come  from  his  lips  with  a  force  which 
no  printed  statement  can  give  them.  Our  experi- 
ence has  been,  that  one  of  the  best  methods  of 
reviewing  a  subject  in  natural  philosophy  is  to  illus- 
trate it  by  some  experiment  not  given  in  the  book, 
and  then  to  question  the  pupils  upon  it.  A  few 
familiar  examples  of  such  experiments  are  given  in 
an  appendix. 

The  subject  has  been  carefully  divided  into  topics, 
and  these  subdivided  into  chapters  and  sections. 
Each  chapter  closes  with  questions,  calculated  to 
test  the  pupil's  knowledge  of  its  contents. 

The  greatest  care  has  been  taken  with  the  cuts 
designed  to  illustrate  the  text ;  and  it  is  believed 
that  they  will  compare  favorably  with  those  in  any 
similar  text-book  yet  published.  Many  of  the  cuts 
are  from  the  French  edition  of  Ganot's  "  Elemen- 
tary Physics." 


CONTENTS. 


I.    MATTER,   FORCE,  AND   MOTION  i 

CHAPTER           I.  MATTER i 

"               II.  THE  STRUCTURE  OF  MATTER      .       .  6 

"              III.  FORCE 16 

"  IV.  THE  PHYSICAL  SCIENCES  ...  20 
"  V.  FIRST  LAW  OF  MOTION  ...  23 
"  VI.  SECOND  LAW  OF  MOTION  ...  28 
"  VII.  THIRD  LAW  OF  MOTION  .  .  34 
"  VIII.  CENTRE  OF  GRAVITY  AND  EQUILI- 
BRIUM    37 

II.    ENERGY  AND   MACHINES    .....  42 

CHAPTER         IX.  WORK  AND  ENERGY    ....  42 

"                 X.  MACHINES 47 

III.  STATES   OF  MATTER 59 

CHAPTER         XI.  FLUIDS 59 

"  XII.  PROPERTIES  OF  GASES,  LIQUIDS,  AND 

SOLIDS 68 

"           XIII.  ATMOSPHERIC  PRESSURE         .       .  78 

IV.  SOUND 86 

CHAPTER      XIV.  ORIGIN  AND  NATURE  OF  SOUND   .  86 

"             XV.  PROPAGATION  OF  SOUND  AND  SYMPA- 
THETIC VIBRATIONS.        ...  91 

V.    HEAT 99 

CHAPTER      XVI.  NATURE  AND  TRANSMISSION  OF  HEAT  99 

"          XVII.  THE  THREE  EFFECTS  OF  HEAT  .       .  104 

"        XVIII.  LATENT  HEAT 112 

V 


VI  CONTENTS. 

PAGE 

VI.    LIGHT 116 

CHAPTER      XIX.  NATURE    AND    TRANSMISSION     OF 

LIGHT 116 

"              XX.  MIRRORS         .                .        .        .  125 

XXI.  LENSES 133 

VII.    ELECTRICITY 144 

CHAPTER    XXII.  FRICTIONAL  ELECTRICITY         .       .  144 

XXIII.  VOLTAIC  ELECTRICITY  .        .       .  156 

VIII.    ELECTRO-MAGNETISM 164 

CHAPTER  XXIV.  ELECTRO-MAGNETS.       ...  164 

"          XXV.  MAGNETO-ELECTRICITY      .       .       .174 

APPENDIX ' 183 


FIRST    BOOK    IN    NATURAL 
PHILOSOPHY. 


I. 

MATTER,    FORCE,    AND    MOTION. 


CHAPTER    I. 

MATTER. 

1.  The  Senses.  —  We  have  five  senses  by  which 
we  perceive  objects  about  us ;  namely,  sight,  hear- 
ing,   touch,    taste,    and   smell.     It    is    by    means   of 
these    senses    that   we    obtain    all    our   knowledge 
of  the  world  around  us. 

2.  Matter.  —  Every   thing   that    occupies    space, 
and  is  capable  of   being  perceived   by  our  senses, 
is  composed  of  matter. 

Matter  may  be  defined  as  that  which  occupies 
space,  and  is  capable  of  being  perceived  by  the 
senses. 

A  body  is  a  distinct  portion  of  matter.  This  term 
is  usually  applied  only  to  those  portions  of  matter 
which  have  a  sensible  size. 

The  different  kinds  of  matter  are  called  sub- 
stances. 

j. 


2  NATURAL  PHILOSOPHY. 

' '  Illustrations.^-  Iron  is  a  substance  ;  and  a  separate  piece 
of  iron  is  a  body.  Wood  is  also  a  substance ;  while  any 
limited  portion  of  wood,  as  a  log,  a  board,  a  box,  or  a  table, 
is  a  body. 

Sometimes  matter  is  in  such  a  position,  or  in 
such  a  condition,  that  we  can  perceive  it  only  by 
one  of  our  senses.  In  other  cases  we  can  perceive 
the  same  matter  by  two  or  more  of  our  senses  at 
once. 

Illustrations.  —  A  star,  owing  to  its  distance,  can  be  per- 
ceived only  by  the  sense  of  sight.  A  distant  beii  can  be 
perceived  only  by  the  sense  of  hearing.  The  air  is  composed 
of  matter  in  such  a  state  that  it  can  be  perceived  only  by 
the  sense  of  touch,  as  when  the  air  is  blown  against  our  hands 
and  face,  or  when  we  move  our  hand  rapidly  to  and  fro  in 
the  air.  We  may  perceive  a  rose  which  we  hold  in  our  hand, 
by  the  sense  of  sight,  of  touch,  and  of  smell  at  one  and 
the  same  time.  An  apple  held  in  the  hand  may  be  perceived 
by  the  sense  of  sight,  of  touch,  of  smell,  and  of  taste. 

3.  The  Dimensions  of   Matter.  —  Since    matter 
occupies   space,    it    must    have    three    dimensions ; 
namely,  length,  breadth,  and  thickness. 

4.  English  Units  of  Length.  —  We  measure  length 
in  inches,  feet,  yards,  and  miles.     These  are  called 
the  English  units  of  length.     It  takes  twelve  inches 
to   make   a  foot,   three   feet   to  make  a  yard,  and 
1760  yards,  or  5280  feet,  to  make  a  mile.     The  inch 
is  usually  divided  into  halves,  quarters,  eighths,  six- 
teenths,  and  thirty-secondths,   each    division    being 
one-half   of   the  next  larger.     Sometimes   the  inch 
is  divided  into  tenths. 

5.  French  Units  of  Length.  —  In  France  lengths 
are  usually  measured  in  meters,  and  in  fractions  or 


NATURAL  PHILOSOPHY.  3 

multiples  of  the  meter.  The  fractions  of  the  meter 
are  the  decimeter,  or  the  tenth  of  the  meter ;  the 
centimeter,  or  the  hundredth  of  the  meter ;  and 
the  millimeter,  or  the  thousandth  of  the  meter. 
The  multiples  of  the  meter  are  the  decameter,  or 
ten  meters  ;  the  hectometer,  or  one  hundred  meters  ; 
and  the  kilometer,  or  one  thousand  meters.  The 
French  units  of  length  in  most  corriVnon  use  are 
the  centimeter,  the  meter,  and  the  kilometer. 

The  meter  is  nearly  forty  inches  (39.37  inches) 
long,  the  decimeter  is  about  four  inches  long,  the 
centimetre  about  four-tenths  or  two-fifths  of  an  inch 
long,  and  the  millimeter  about  four  one-hundredths 
or  one  twenty-fifth  of  an  inch  long.  The  kilometer 
is  about  five-eighths  of  a  mile. 

6.  Units  of   Surface. — The  units  in  which   sur- 
faces  are    measured    are    the  squares  of    the  units 
of  length. 

The  English  units  of  surface  are  the  square  inch, 
the  square  foot,  the  square  yard,  and  the  square 
mile.  There  are  144  square  inches  in  a  square 
foot,  and  nine  square  feet,  or  1296  square  inches, 
in  a  square  yard. 

The  French  units  of  surface  are  the  square  meter, 
the  square  decimeter,  the  square  centimeter,  etc. 
There  are  about  1550  square  inches  in  a  square 
meter. 

7.  Units  of  Volume.  —  By  the  volume  of  a  body 
we  mean  the  space  which   it  occupies.     The  units 
of  volume  are  the  cubes  of  the  units  of  length. 

The  English  units  of  volume  are  the  cubic  inch, 
the  cubic  foot,  the  cubic  yard,  etc.  There  are  1728 


4  NATURAL  PHILOSOPHY. 

cubic   inches   in    a   cubic   foot,    and    27  cubic   feet 
in  a  cubic  yard. 

The  French  units  of  volume  are  the  cubic  meter, 
the  cubic  decimeter,  the  cubic  centimeter,  etc.  The 
cubic  meter  is  equal  to  nearly  thirty-six  cubic  feet. 
The  cubic  decimeter  is  called  a  liter.  The  liter 
is  equal  to  about  one  pint  and  three-fourths. 

8.  Mass.  —  By  the  mass  of  a  body  we  mean  the 
quantity  of    matter  in  it.     Bodies  which   have  the 
same    volume    often    have    very    different    masses. 
Thus   a   cubic  inch  of   cork  has  the  same  volume 
as  a   cubic   inch   of    lead ;    but   the   cubic   inch   of 
lead  has  a  much  greater  mass  than  the  cubic  inch 
of  cork,   the  lead   being  much   more  compact,   or 
dense,  than  the  cork. 

9.  Units  of  Mass.  —  We  measure  mass  in  pounds, 
ounces,   and  grains.      It   takes   sixteen   ounces,   or 
seven  thousand  grains,  to  make  a  pound.     A  pound 
is   simply  an   amount   of   matter  equal   to   that   in 
a  certain  piece  of   platinum  preserved  by  the  gov- 
ernment as  a  standard. 

The  French  unit  of  mass  is  called  the  gram.  It 
is  the  amount  of  matter  in  a  cubic  centimeter  of 
water  at  a  temperature  of  thirty-nine  degrees.  A 
cubic  centimeter  of  water  is  a  little  cube  of  water 
which  measures  about  two-fifths  of  an  inch  each 
way.  The  gram  is  equal  to  about  fifteen  grains. 
There  are  about  four  hundred  and  sixty  grams  in 
a  pound.  The  subdivisions  of  the  gram  are  the 
decigram,  the  centigram,  and  the  milligram.  These 
are  respectively  the  tenth,  the  hundredth,  and 
the  thousandth  of  a  gram.  The  multiples  of  the 


NATURAL  PHILOSOPHY.  5 

gram  are  the  decagram,  the  hectogram,  and  the  kilo- 
gram. These  are  respectively  ten,  a  hundred,  and  a 
thousand  grams.  A  kilogram  is  nearly  two  pounds 
and  a  quarter. 

NOTE. —  In  scientific  books  the  French  forms,  metre,  deci- 
metre, gramme,  kilogramme,  litre,  etc.,  are  often  used  instead 
of  the  English  forms  given  above.  Decameter,  hectogram,  etc., 
are  sometimes  spelled  dekameter,  hektogram,  etc. 

QUESTIONS. 

i.  How  many  senses  have  we?  2.  Name  them.  3.  What 
do  we  do  by  our  senses  ?  4.  What  do  we  obtain  by  means  of 
them?  5.  What  is  matter?  6.  What  is  meant  by  a  body? 
7.  Give  illustrations.  8.  Give  illustrations  of  matter  so  situ- 
ated that  we  can  perceive  it  by  one  of  our  senses  only.  9.  Give 
an  illustration  of  matter  which  exists  in  such  a  condition  that 
we  can  perceive  it  by  one  sense  only.  10.  Give  illustrations 
of  matter  which  may  be  jperceived  by  several  of  the  senses 
at  the  same  time.  11.  How  many  dimensions  has  matter? 
12.  Name  them.  13.  Name  the  English  units  by  which  length 
is  measured.  14.  How  many  inches  in  a  foot  ?  15.  How  many 
feet  in  a  yard?  16.  How  many  inches  in  a  yard?  17.  How 
many  yards  in  a  mile?  18.  How  many  feet  in  a  mile?  19. 
What  are  the  usual  subdivisions  of  the  inch  ?  20.  What  is 
the  French  unit  of  length?  21.  Give  the  subdivisions  and 
multiples  of  this  unit.  22.  About  how  many  inches  in  a  meter  ? 
23.  In  a  decimeter?  24.  In  a  centimeter?  25.  In  a  milli- 
meter? 26.  What  is  about  the  length  of  a.  kilometer?  27. 
What  is  the  exact  length  of  the  meter  ?  28.  What  are  units 
of  surface?  29.  Name  the  chief  English  units  of  surface. 
30.  Name  the  chief  French  units  of  surface.  31.  How  many 
square  inches  in  a  square  foot  ?  32.  How  many  square  feet  in 
a  square  yard?  33.  How  many  square  inches  in  a  square 
yard?  34.  How  many  square  inches  in  a  square  meter?  35. 
What  are  units  of  volume  ?  36.  What  is  meant  by  the  volume 
of  a  body  ?  37.  Name  the  chief  English  units  of  volume. 


6  NATURAL   PHILOSOPHY. 

38.  Name  the  chief  French  units  of  volume.  39.  How  many 
cubic  inches  in  a  cubic  foot  ?  40.  How  many  cubic  feet  in  a 
cubic  yard?  41.  How  many  cubic  feet  in  a  cubic  meter? 
42.  What  is  the  liter?  43.  What  is  its  volume  in  English 
measure?  44.  What  is  meant  by  the  mass  of  a  body?  45. 
What  is  the  difference  between  mass  and  volume?  46.  Do 
bodies  of  the  same  volume  always  have  the  same  mass  ?  47. 
Give  an  illustration.  48.  Name  the  English  units  of  mass. 
49.  How  many  ounces  in  a  pound  ?  50.  How  many  grains  in 
a  pound?  51.  What  is  the  pound ?  52.  What  is  the  name  of 
the  French  unit  of  mass?  53.  What  is  it?  54.  About  how 
many  grains,  in  the  gram  ?  55.  Name  the  subdivisions  and  the 
multiples  of  the  gram.  56.  How  many  pounds  in  a  kilogram  ? 


CHAPTER    II. 

THE   STRUCTURE   OF   MATTER. 

10.  The  Material  Universe.  —  All  the  matter  in 
existence   constitutes   what    is    called    the   material 
universe. 

11.  The  Structure  of  the  Material  Universe. — 
The  material  universe  is  not  a  continuous  mass  of 
matter,  but  is  composed  of  bodies  like  the  earth, 
the  moon,  the  sun,  and  the  stars  ;  and  these  bodies, 
instead  of  being  in  contact,  are  very  far  apart.     They 
are  not  scattered  at  random  through  space,  but  are 
grouped  together  in  various*  ways  so  as  to  form  sys- 
tems of  bodies. 

The  sun  is  accompanied  by  a  number  of  bodies 
similar  to  the  earth,  which  are  called  planets.  The 
five  planets,  which,  next  to  the  earth,  are  best 
known,  are  Mercury,  Venus,  Mars,  Jupiter,  and  Sat- 
urn. These  planets  all  revolve  around  the  sun  ;  that 


NATURAL  PHILOSOPHY.  7 

is  to  say,  they  move  around  the  sun  in  paths,  or 
orbits,  which  are  of  a  circular  form.  The  earth 
moves  in  her  orbit  at  the  rate  of  eighteen  miles  a 
second ;  Mercury,  at  the  rate  of  nearly  thirty  miles 
a  second ;  and  Saturn,  the  slowest  of  the  planets 
named,  at  the  rate  of  about  six  miles  a  second. 
An  express  train  at  its  fastest  goes  about  a  mile 
a  minute.  The  earth  is  travelling  in  her  orbit  at 
the  rate  of  more  than  a  thousand  miles  a  minute. 
An  express  train,  moving  as  fast  as  the  earth 
would  go  from  New  York  to  Chicago  in  about 
one  minute,  from  Chicago  to  San  Francisco  in 
about  two  minutes,  and  around  the  globe  in  twenty- 
five  minutes. 

Many  of  the  planets  are  accompanied  by  moons. 
The  earth  has  one  moon,  Mars  has  two  moons, 
Jupiter  four,  and  Saturn  eight.  Mercury  and  Venus 
are  without  moons.  The  moons  revolve  around 
their  planets,  and  always  keep  with  them  in  their 
revolutions  around  the  sun.  A  planet  and  its 
moons  constitute  a  group  of  bodies  called  a  plane- 
tary system. 

The  sun  is  moving  through  space  at  the  rate  of 
about  two  hundred  and  fifty  miles  a  minute,  or  about 
one-fourth  as,  fast  as  the  earth  is  moving  around 
the  sun.  The  planets  which  revolve  around  the  sun 
always  keep  with  him  in  his  journey  through  space. 
The  sun  and  planets  constitute  a  group  of  bodies 
called  a  solar  system.  A  solar  system  is  thus  seen 
to  be  made  up  of  single  planets  and-  of  planetary 
systems.  The  structure  of  our  solar  system  as 
described  above  is  shown  in  Figure  i. 


NATURAL   PHILOSOPHY. 


Fig.  i. 


NATURAL  PHILOSOPHY.  9 

Our  sun  is  simply  one  of  the  stars  ;  and,  at  the 
distance  of  the  other  stars,  it  would  appear  smaller 
than  many  of  them.  Each  star  is  probably  the 
centre  of  a  solar  system  similar  to  our  own.  These 
solar  systems,  singly  and  in  groups,  make  up  the 
stellar  universe ;  just  as  the  planets,  singly  and  in 
groups,  make  up  the  solar  system. 

12.  The  Dimensions  of  the  Stellar  Universe. — 
We  have  seen  that  a  body  moving  with  the  speed 
of  the  earth  in  its  orbit  would  go  around  the  globe 
in  twenty-five  minutes.     Light  travels    at   the  rate 
of  nearly  a  hundred   and   ninety  thousand    miles  a 
second,  or  fast  enough  to  go  around  the  world  more 
than  seven  times  in  a  second.     It  would  take  light 
about  one  second  and  a  fourth  to  go  from  the  earth 
to  the  moon,   about   eight  minutes  to  go  from  the 
earth  to  the  sun,  and  about  three  years  and  a  half 
to  go  from  the  sun  to  the  nearest  star.     We  thus 
gain  some  faint  notion  of  the  distance  which  sepa- 
rates moon  from  planet,  planet  from  sun,  and   sun 
from  star. 

13.  Molar  Motion. — The  motion  of  visible  bodies 
is  called  molar  motion. 

Illustrations.  —  We  have  examples  of  molar  motion  in  the 
fall  of  a  stone,  in  the  flight  of  a  bird,  in  the  sailing  of  a  ship, 
and  in  all  the  motion  of  bodies  at  the  surface  of  the  earth. 
The  grandest  examples  of  molar  motion  are  exhibited  in  the 
stellar  universe.  The  moons  are  at  one  and  the  same  time 
moving  in  their  orbits  around  their  planets,  with  their  planets 
around  the  sun,  and  with  the  sun  through  space. 

Every  body  in  the  universe  of  which  we  have 
any  knowledge  is  in  motion,  and  very  many  bodies 


10  NATURAL  PHILOSOPHY. 

share  several  very  rapid  motions  at    one   and   the 
same  time. 

14.  Rest.  —  Bodies    are  said  to  be  at  rest  when 
they  are  not  changing  their  positions  with  respect 
to  other  bodies  around  them,  even  though  they  are 
really  moving  very  rapidly. 

Illustrations,  —  A  body  lying  on  the  deck  of  a  steamer  is 
said  to  be  at  rest,  because  ij  keeps  in  the  same  position  with 
respect  to  other  objects  cm  the  steamer.  It  may,  however,  be 
moving  forward  very  rapidly  with  the  steamer  through  the 
water.  A  body  lying  on  the  ground  is  said  to  be  at  rest, 
because  it  keeps  in  the  same  position  with  reference  to  sur- 
rounding points  on  the  earth's  surface  ;  but  it  is  really  moving 
forward  very  rapidly  with  the  earth  through  space. 

15.  The  Structure  of  Bodies.  —  The  structure  of 
a  body  is  now  generally  held  to  be  similar  to  that 
of  the  stellar  universe.     A  body  is   not  a  continu- 
ous, uninterrupted  mass  of  matter,  but  is  made  up 
of  a  number  of  very  minute  and  distinct  particles, 
called  atoms.     These  atoms  are  arranged,  in  various 
ways,  into  groups,  called  molecules.     The  atoms  cor- 
respond   to    the    sun,    moon,   and    planets,   and    the 
molecules    to    the   solar  systems.     A  body  is  made 
up  of    these   molecules  in   the    same   way  that    the 
stellar  universe  is  made  up  of  solar  systems.     The 
spaces  between  the  atoms  and  molecules  within  a 
body  are  probably  as  great,  compared  with  the  size 
of    the   atoms    and    molecules,    as    are    the    spaces 
between  the  planets,  sun,  and  stars,  compared  with 
the  size   of   these  bodies.     If   a  being  were   living 
on  one  of  the  atoms  within  a  body,   as  small  com- 
pared with  the  atom   as-  we  are   compared  with  the 


NATURAL  PHILOSOPHY.  II 

earth,  the  atoms  and  molecules  about  him,  were 
they  visible,  would  appear  as  far  off  to  him  as  the 
sun,  moon,  planets,  and  stars  do  to  us. 

16.  The  Dimensions  of  the  Atoms  and   Mole- 
cules. —  No    microscope    has    ever   yet    been    con- 
structed powerful  enough  to   enable  us   to  see  the 
spaces  between  the  molecules,  much  less  the  mole- 
cules themselves  and  ^the  atoms  of  which  they  are 
composed. 

IllustKations.  —  It  has  been  estimated,  that  were  a  drop  of 
water  enlarged  to  the  size  of  the  earth,  all  its  molecules  being 
enlarged  in  the  same  proportion,  the  molecules  would  be  about 
as  large  as  billiard-balls ;  that  is  to  say,  a  molecule  of  water 
is  as  small,  in  comparison  with  a  billiard-ball,  as  a  drop  of 
water  in  comparison  with  the  earth.  It  is  supposed  that  there 
are  at  least  three  hundred  quintillions  of  molecules  in  one  cubic 
inch  of  air,  —  a  number  which  would  be  represented  by  three 
followed  by  twenty  ciphers.  At  the  same  time  it  is  believed 
that  the  molecules  themselves  occupy  only  one  three-thousandth 
of  the  space  in  the  cubic  inch.  The  atoms  that  make  up  the 
molecules  are  also  believed  to  be  very  far  apart  compared  with 
their  size.  We  thus  gain  some  notion  of  the  extreme  fineness 
of  the  atomic  dust  of  which  matter  is  composed. 

17.  Molecular   Motion.  —  The    atoms   and  mole- 
cules of  a  body  are  in  incessant  motion,  as  well  as 
the  planets  and  the  solar  systems.     The  atoms  are 
all   the  time  moving  to   and  fro  within    the  mole- 
cules, and  the  molecules  to  and  fro  within  the  body. 
The  motions  of   the  atoms  and  molecules  within  a 
body  are  called  molecular  motions. 

1 8.  The  Ether.  —  All  the  spaces  between  moons, 
planets,  and  stars   in   the   stellar  universe,   and    all 
the  spaces  among    the    atoms    and    molecules    of   a 


12  NATURAL  PHILOSOPHY. 

body,  are  supposed  to  be  filled  with  a  very  fine  and 
delicate  material  called  the  ether. 

Illustrations.  —  Fill  a  glass  vessel  with  a  mixture  of  shot 
and  marbles,  and  then  pour  into  it  all  the  water  it  will  hold. 
The  water  will  completely  fill  all  the  spaces  among  the  shot  and 
marbles.  All  the  atoms  and  molecules  of  matter  are  immersed 
in  the  ethereal  ocean  in  the  same  way  that  the  shot  and  mar- 
bles are  immersed  in  the  water.  Of  course  the  atoms  and 
molecules  are  comparatively  much  farther  apart  than  the  shot 
and  marbles. 

19.  Porosity   of   Matter.  —  All    substances    are 
porous ;  that  is  to  say,  in  every  part  of  them  there 
are   spaces  which   are   not   occupied  with  material 
particles.     When  these  spaces  are  large  enough  to 
be   seen,  they  are  called  visible  pores  ;  and,  when 
they  are  too  small  to  be  seen  even  with  a  micro- 
scope, they  are  called  physical  pores. 

Illustrations.  —  A  piece  of  wood  is  full  of  visible  pores. 
Pour  a  little  quicksilver  upon  a  piece  of  chamois-skin,  and 
gather  up  the  skin  so  as  to  form  a  kind  of  bag  enclosing 
the  mercury.  On  squeezing  this  bag  you  will  see  the  quick- 
silver running  through  the  pores  of  the  skin  in  a  number  of 
fine  streams.  In  1661  some  learned  men  in  Florence  filled  a 
thin  hollow  globe  of  gold  with  water,  and,  after  closing  the 
opening  perfectly  tight,  they  subjected  the  globe  to  great 
pressure.  The  water  came  through  the  pores  of  the  gold,  and 
wet  the  outside  of  the  globe. 

20.  Compressibility  of  Matter.  —  A  body  is  said 
to   be  compressed  when   it   is   made   to  occupy  less 
space.     All  substances  are  compressible.     When  a 
body  is  compressed,  its  molecules  are  brought  nearer 
together. 


NATURAL   PHILOSOPHY.  13 

Illustrations.  —  Hold  a  glass  goblet  mouth  downward,  and 
force  it  down  into  water.  The  water  will  rise  a  little  way  into 
the  goblet,  the  air  inside  becoming  more  and  more  compressed 
as  the  vessel  is  forced  down. 

21.  Impenetrability  of  Matter.  —  When    we    say 
that  matter  is  impenetrable,  we  mean   that   no  two 
portions  of  matter  can   occupy  the   same  space  at 
the  same  time.     All  substances  are  impenetrable. 

Illustrations.  —  Fill  a  goblet  to  the  brim  with  water,  and 
drop  a  marble  into  it.  Some  of  the  water  will  overflow.  The 
marble  can  enter  the  goblet  only  by  displacing  a  bulk  of  water 
equal  to  its  own.  When  we  pushed  the  inverted  goblet  down 
into  water,  the  liquid  rose  only  a  little  way  into  it,  because  the 
goblet  was  already  filled  with  air.  The  water  rose  a  little  way 
because  the  air  was  compressed.  When  we  pour  water  into 
an  upright  goblet,  the  air  is  displaced  as  rapidly  as  the  water 
enters.  Water  cannot  be  poured  into  a  fine  tube  which  is 
closed  at  the  bottom,  because  there  is  not  sufficient  room  for 
the  water  to  enter  and  the  air  to  escape  at  the  same  time. 

Strictly  speaking,  it  is  only  the  atoms  of  matter 
that  are  impenetrable.  In  certain  cases,  the  mole- 
cules of  one  substance  may  work  their  way  in 
among  those  of  another,  and  occupy  the  space  be- 
tween those  molecules.  Also,  in  certain  cases,  the 
atoms  of  one  substance  may  be  introduced  into  the 
spaces  between  the  atoms  in  the  molecules  of  an- 
other substance. 

22.  Divisibility  of  Matter.  —  We  may  divide  and 
subdivide  any  body,  as  a  piece  of  wood,  into  smaller 
and  smaller  pieces,  until  these  become  too  small  to 
be  seen  with  the  unaided   eye  ;  and  then,  with  the 
microscope,  we   may  continue  the  division   till   the 
pieces  are  too  small  to  be  seen  with  the  microscope. 


14  NATURAL  PHILOSOPHY. 

Were  the  microscope  powerful  enough,  we  might  go 
on  dividing  the  body  until  we  reached  the  molecules. 
During  all  this  time  the  substance  of  all  these  pieces 
would  remain  exactly  the  same  as  that  of  the  origi- 
nal body.  Were  the  microscope  powerful  enough, 
and  the  instrument  at  our  disposal  delicate  enough, 
to  enable  us  to  divide  the  molecules,  the  original 
substance  would  be  destroyed,  and  we  should  obtain 
new  substances. 

By  certain  processes  we  may  separate  a  substance 
into  its  molecules,  and  break  up  the  molecules  them- 
selves into  their  atoms  ;  but  we  have  found  no 
means  of  dividing  an  atom. 

Illustrations.  —  When  water  evaporates,  it  is  resolved  into 
its  molecules,  which  then  fly  about  loosely  in  the  air.  When 
the  vapor  of  water  is  heated  to  a  very  high  temperature,  the 
molecules  themselves  are  broken  up  into  their  atoms ;  and,  as 
there  are  two  kinds  of  atoms  in  the  molecules  of  water,  we 
obtain  two  new  substances,  called  hydrogen  and  oxygen. 

23.  Indestructibility  of  Matter.  —  Matter  may 
be  made  to  assume  a  great  variety  of  forms ;  but  no 
portion  of  matter  can  be  blotted  out  of  existence 
except  by  the  power  which  created  it.  Bodies  may 
be  crushed  into  the  finest  powder  so  as  to  destroy 
their  form,  and  their  molecules  may  be  broken  up 
so  as  to  destroy  their  substance ;  but  the  atoms  are 
indestructible  by  any  means  known  to  us. 

QUESTIONS. 

i.  What  is  meant  by  the  material  universe?  2.  Of  what  is 
this  universe  composed  ?  3.  How  are  the  bodies  which  com- 
pose the  material  universe  arranged?  4.  By  what  is  the  sun 


NATURAL   PHILOSOPHY.  15 

accompanied?  5.  Name  the  five  principal  planets.  6.  What 
motion  have  the  planets  ?  7.  How  fast  do  the  earth,  Mercury, 
and  Saturn  move  in  their  orbits  ?  8.  How  does  the  Dearth's 
speed  compare  with  that  of  an  express  train  ?  9.  By  what  are 
many  of  the  planets  accompanied?  10.  Which  planets  have 
moons?  ii.  How  many  moons  has  each  ?  12.  Which  planets 
are  without  moons ?  13.  What  motion  have  the  moons?  14. 
What  constitutes  a  planetary  system  ?  15.  What  motion  has 
the  sun?  16.  What  constitutes  a  solar  system?  17.  What  is 
probably  true  of  the  stars?  18.  What  constitutes  the  stellar 
universe?  19.  How  fast  does  light  travel?  20.  How  long 
would  it  take  light  to  go  from  the  earth  to  the  moon  ?  21. 
From  the  earth  to  the  sun  ?  22.  From  the  sun  to  the  nearest 
star?  23.  What  is  meant  by  molar  motion?  24.  Give  some 
examples  of  molar  motion.  25.  What  motion  have  the  moons  ? 
26.  What  is  true  of  every  body  in  the  universe  as  regards 
motion  ?  27.  When  is  a  body  said  to  be  at  rest  ?  28.  Give 
illustrations  of  bodies  in  motion  which  are  said  to  be  at  rest. 
29.  To  what  is  the  structure  of  a  body  similar  ?  30.  Of  what 
are  bodies  made  up ?  31.  What  are  molecules?  32.  To  what 
do  the  atoms  and  molecules  of  a  body  correspond  ?  33.  What 
is  the  size  of  the  spaces  between  the  atoms  and  molecules, 
compared  with  the  sizes  of  the  atoms  and  molecules  ?  34. 
How  far  off  would  the  surrounding  atoms. and  molecules  appear 
to  a  being  small  enough  to  live  on  one  of  the  atoms?  35. 
Can  the  molecules  be  seen  with  a  microscope  ?  36.  Give  the 
illustration  of  the,  .drop  of  water.  37.  How  many  molecules 
are  there  in  a  cubic  inch  of  air  ?  38.  What  fraction  of  the 
space  is  occupied  by  these  molecules?  39.  What  is  meant 
by  molecular  motion  ?  40.  Describe  the  motion  of  the  atoms 
and  molecules.  41.  What  is  meant  by  the  ether?  42.  Give 
the  illustration  of  the  marbles,  shot,  and  water.  43.  What  do 
we  mean  when  we  say  that  matter  is  porous  ?  44.  Name  and 
describe  the  two  kinds  of  pores  that  are  contained  in  bodies. 
45.  Give  illustrations.  46.  What  do  we  mean  when  we  say- 
that  matter  is  compressible  ?  47.  What  takes  place  when  mat- 
ter is  compressed?  48.  Give  illustrations.  49.  What  do  we 
mean  when  we  say  that  matter  is  impenetrable?  50.  Give 


16  NATURAL   PHILOSOPHY. 

illustrations.  51.  What  portions  of  matter  only  are  really  im- 
penetrable ?  52.  To  what  extent  is  matter  divisible  ?  53.  To 
what  extent  may  the  division  of  a  body  be  carried  without 
altering  its  substance  ?  54.  Are  the  atoms  divisible  ?  55. 
What  takes  place  when  water  evaporates?  56.  When  the 
vapor  of  water  is  heated  to  a  very  high  temperature?  57. 
What  do  we  mean  when  we  say  that  matter  is  indestructible  ? 
58.  What  portions  of  matter  only  are  indestructible  ? 


CHAPTER    III. 

FORCE. 

24.  Definition    of    Force.  —  Any  push    or   pull, 
of   whatever  origin,  upon  matter,   is  called  a  force. 
A  pulling  force  is  called  an  attractive  force,  and  a 
pushing  force  a  repulsive  force.     Forces  always  act 
between  two  portions  of  matter.     We  do  not  know 
where    the    force    resides,    but    we    usually    speak 
of   it  as   residing  in    the   portions   of   matter  acted 
upon. 

Illustrations.  —  The  action  of  an  attractive  force  may  be 
illustrated  by  fastening  a  ball  to  each  end  of  a  rubber  cord, 
and  then  pulling  the  balls  apart.  The  cord  pulls  upon  each  of 
the  balls,  and  tends  to  draw  them  together.  The  action  of  a 
repulsive  force  may  be  illustrated  by  placing  a  ball  on  each 
side  of  a  thick  piece  of  rubber,  and  then  crowding  the  balls 
together.  The  compressed  rubber  pushes  upon  each  of  the 
balls,  and  tends  to  drive  them  apart. 

25.  The  Three  Great  Forces  of  Nature.  — The 
three  great  forces  of    Nature  are  gravity,  cohesion, 
and  affinity. 

Gravity  is   the  force   which   holds  bodies   to   the 


NATURAL  PHILOSOPHY.  I? 

earth,  moons  to  their  planets,  and  planets  to  the 
sun.  It  is  an  attractive  force,  which  tends  to  draw 
bodies  together,  and  which  acts  through  all  known 
distances.  It  is  a  molar  force. 

Cohesion  is  the  force  which  holds  together  the 
molecules  of  a  body.  It  is  an  attractive  force,  which 
tends  to  draw  molecules  together.  It  is  much 
stronger  than  gravity,  but  it  acts  only  through 
insensible  distances.  It  is  a  molecular  force. 

Illustrations. —  It  is  gravity  that  holds  a  rod  of  iron  to  the 
table,  and  cohesion  that  holds  the  rod  together.  It  is  much 
easier  to  lift  the  rod  from  the  table  than  to  pull  it  in  two.  This 
shows  that  cohesion  is  stronger  than  gravity.  Were  the  rod 
once  broken,  so  as  to  separate  the  molecules  ever  so  little, 
the  parts  could  then  be  separated  with  ease.  This  shows  that 
cohesion  acts  only  through  insensible  distances. 

Affinity  is  the  force  that  holds  together  the  atoms 
in  the  molecules.  It  is  a  stronger  force  than  cohe- 
sion, but  it  does  not  act  through  so  great  distances. 
It  is  an  atomic  force. 

26.  Stress,  Action,  and  Reaction.  —  The  whole 
action  of  a  force  between  two  portions  of  matter 
is  called  a  stress.  Every  stress  is  made  up  of  two 
pushes  or  two  pulls,  one  upon  each  of  the  portions 
of  matter.  These  are  called  the  two  aspects  of  the 
stress.  We  always  think  of  the  force  as  residing 
in  the  portion  of  matter  acted  upon,  and  speak  of 
the  two  portions  of  matter  as  pushing  or  pulling 
each  other.  We  call  one  of  these  pushes  or  pulls 
an  action,  and  the  other  a  reaction.  Whether  we 
call  the  one  or  the  other  of  these  the  action  depends 
upon  the  way  we  are  looking  at  it. 


1 8  NATURAL   PHILOSOPHY. 

Illustrations.  —  The  whole  pull  of  gravity  between  a  stone 
and  the  earth  is  called  a  stress.  We  say  that  the  stone  and  the 
earth  pull  each  other.  When  we  think  of  the  earth  as  pulling 
the  stone,  we  call  the  pull  upon  the  stone  the  action  of  the 
earth  upon  the  stone,  and  then  we  call  the  pull  upon  the  earth 
the  reaction  of  the  stone  upon  the  earth.  When  we  think  of 
the  stone  as  pulling  the  earth,  we  call  the  pull  upon  the  earth 
the  action  of  the  stone  upon  the  earth,  and  then  we  call  the 
pull  upon  the  stone  the  reaction  of  the  earth  upon  the  stone. 

When  a  cannon  is  fired,  there  is  a  repulsive  force  acting 
between  the  cannon  and  the  ball.  The  whole  action  of  this 
force  is  called  a  stress.  When  we  think  of  the  cannon  as 
pushing  the  ball,  we  call  the  push  upon  the  ball  the  action  of 
the  cannon  upon  the  ball,  and  then  we  call  the  push  upon  the 
cannon  the  reaction  of  the  ball  upon  the  cannon.  When  we 
think  of  the  ball  as  pushing  the  cannon,  we  call  the  push  upon 
the  cannon  the  action  of  the  ball  upon  the  cannon,  and  then 
we  call  the  push  upon  the  ball  the  reaction  of  the  cannon  upon 
the  ball. 

27.  Strain.  —  When  the  shape  or  size  of  a  body 
is  altered  under  the  action  of  a  force,  the  body  is 
said  to  be  strained.     A  strain  is  any  distortion,  of 
whatever   kind,    which    is    maintained    by   a    force. 
There  are  strains  of  flexure  (bending),  of    traction 
(drawing    out),    of    torsion    (twisting),    and    of    com- 
pression. 

Illustrations.  —  When  a  strip  of  whalebone  is  held  bent,  or 
a  watch-spring  is  coiled  up,  it  is  said  to  be  strained j  and  its 
strain  is  that  of  flexure.  When  a  piece  of  rubber  is  stretched, 
it  is  said  to  be  strained;  and  its  strain  is  that  of  traction. 
When  a  strip  of  whalebone  is  twisted,  it  is  strained;  and  the 
strain  is  that  of  torsion.  When  air  is  compressed,  as  in  the 
experiment  of  pushing  an  inverted  goblet  into  water,  the  strain 
upon  the  air  is  that  of  compression. 

28.  Elasticity.  —  Every  strained    body   tends    to 


NATURAL  PHILOSOPHY.  19 

recover  its  original  shape  or  volume.  This  tendency 
of  a  strained  body  to  recover  itself  is  called  elas- 
ticity. There  are  four  varieties  of  elasticity,  corre- 
sponding to  the  four  varieties  of  strain  ;  namely, 
elasticity  of  flexure,  of  traction,  of  torsion,  and  of 
compression. 

Illustrations.  —  Bend  or  twist  a  whalebone.  On  releasing  it, 
we  discover  its  tendency  to  recover  its  original  shape.  So,  too, 
on  stretching  and  releasing  a  piece  of  rubber,  we  see  its  ten- 
dency to  recover  its  original  length.  On  removing  an  inverted 
goblet  which  has  been  plunged  into  water,  we  discover  the  ten- 
dency of  the  compressed  air  to  recover  its  original  volume. 

29.  Measurement  of  Forces.  —  A  force  always 
tends  to  change  the  velocity  of  the  body  on  which 
it  acts. 

Illustrations.  —  When  a  stone  is  thrown  upward,  gravity 
makes  it  move  slower  and  slower ;  and,  when  a  body  is  falling, 
gravity  makes  it  move  faster  and  faster. 

Forces  are  measured,  either  by  comparing  them 
with  the  pull  of  gravity,  or  by  the  change  of  velocity 
which  they  are  capable  of  producing  in  a  second. 

The  pull  of  gravity  upon  a  mass  of  a  pound  is 
'called  a  pound ;  the  pull  of  gravity  upon  a  mass  of 
a  grain  is  called  a  grain ;  and  the  pull  of  gravity 
upon  a  mass  of  a  gram  is  called  a  gram.  A  force 
of  a  pound  is  a  force  equal  to  the  pull  of  gravity 
upon  a  mass  of  a  pound ;  and  a  force  of  a  grain  or 
a  gram  is  a  force  equal  to  the  pull  of  gravity  upon 
a  mass  of  a  grain  or  a  gram.  A  force  of  ten  grains 
is  one  equal  to  ten  times  the  pull  of  gravity  upon 
a  mass  of  a  grain. 

A   force   capable  of   changing   the  velocity  of   a 


20  NATURAL  PHILOSOPHY. 

pound  of  mass  one  foot  in  a  second  —  that  is,  to 
make  the  mass  move  one  foot  a  second  faster  or 
slower  —  is  called  a  poundal  of  force.  A  force  of  a 
poundal  is  nearly  equal  to  the  pull  of  gravity  upon 
half  an  ounce. 

QUESTIONS. 

i.  What  do  we  mean  by  a  force?  2.  Name  the  two  kinds 
of  force.  3.  Give  an  illustration*  of  each.  4.  Name  the  three 
great  forces  of  Nature.  5.  Give  an  account  of  gravity.  6.  Of 
cohesion.  7.  Of  affinity.  8.  What  do  we  mean  by  a  stress  ? 
9.  What  two  aspects  has  every  stress?  10.  Where  do  we 
always  think  of  the  force  as  residing  ?  n.  What  names  do  we 
give  to  the  two  aspects  of  a  stress?  12.  Illustrate  the  use  of 
these  terms  in  the  case  of  gravity  acting  between  the  earth  and 
a  stone.  13.  In  the  case  of  a  cannon.  14.  When  is  a  body 
said  to  be  strained?  15.  Define  a  strain.  16.  Name  the  dif- 
ferent kinds  of  strain.  17.  Give  an  illustration  of  each.  18. 
What  does  every  strained  body  tend  to  do?  19.  What  name 
do  we  give  to  this  tendency?  20.  Name  the  different  kinds  of 
elasticity.  21.  Illustrate  each.  22.  What  does  a  force  always 
tend  to  do  to  a  body  ?  23.  What  do  we  mean  by  a  force  of  a 
pound?  24.  Of  five  pounds  ?  25.  Of  a  grain?  26.  Of  a  gram? 
27.  Of  fifteen  grams  ?  28.  What  do  we  mean  by  a  poundal  oi 
force?  29.  It  is  about  equal  to  what  pull  of  gravity?  30. 
What  should  we  mean  by  fifty  poundals  of  force  ? 


CHAPTER    IV. 

THE    PHYSICAL   SCIENCES. 

30.   Phenomenon.  —  Any  manifestation  or  occur- 
rence is  called  a  phenomenon. 

Illustrations.  —  The  shining  of  a  star,  the  falling  of  a  stone 
and  the  growth  of  a  plant,  are  all  phenomena. 


NATURAL  PHILOSOPHY.  21 

31.  The  Physical  Sciences.  —  The  physical  sci- 
ences treat  of  matter  and  force  irrespective  of  the 
phenomena  of  life.     The  chief  physical  sciences  are 
mechanics,  astronomy,  physics,  and  chemistry. 

32.  Material  Units. — A  material  unit  is  either 
a  single  mass,  or  else  a  group  of  masses,  which  keep 
together   in    their   motions.     The    three    orders   of 
material  units  are  bodies,  molecules,  and  atoms.     A 
body  is  a  group  of  molecules  which  keep  together 
in  all  their  motions,  and  a  molecule  is  a  group  of 
atoms  which  keep  together  in  all  their  motions. 

33.  Mechanics.  —  Mechanics   is    that    branch    of 
physical  science  which  treats  of  the  action  of  force, 
and  of  the  laws  of  motion,  irrespective  of  any  par- 
ticular order  of  material  units. 

34.  Astronomy.  —  Astronomy  treats  of  the  heav- 
enly  bodies,   of    gravity,   by  which   the   motion   of 
these  bodies  is  regulated,  and  of   the  structure  of 
the  heavens. 

35.  Chemical    and    Physical    Properties.  —  The 
peculiarities,  or  properties,  of  matter  which  are  due 
to  the  action  of  affinity  and  to  the  atomic  structure 
of    the    molecules    are    called    chemical  properties. 
Those   properties  which  result  from  the  action  of 
cohesion  and  the   molecidar  structure   of   the   body 
are  called  physical  properties. 

36.  Physical     and     Chemical    Changes.  —  Any 
change  in  matter  which  alters  the  atomic  structure 
of  the  molecules,  and  so  produces  a  change  of  sub- 
stance, is    called    a   chemical  change.     Any    change 
which  leaves  the  molecules  intact,  and  which  does 
not  alter  the  substance,  is  called  a  physical  change. 


22  NATURAL   PHILOSOPHY. 

Illustrations.  —  When  ice  melts,  or  water  boils,  the  molecu- 
lar structure  of  the  body  is  altered,  but  the  molecules  them- 
selves remain  intact.  There  is  no  change  of  substance. 
Melting  and  boiling  are  physical  changes.  When  a  piece  of 
iron  rusts,  the  old  molecules  are  broken  up,  and  new  molecules 
are  formed.  There  is  a  change  of  substance.  The  rust  is  a 
different  substance  from  the  iron.  The  rusting  of  iron  is 
a  chemical  change. 

37.  Physics.  —  Physics  treats  of  molecules,  of  the 
molecular   structure   of    bodies,  the   motion  of   the 
molecules,  the  action  of  cohesion,  and  of  physical 
properties  and  changes. 

38.  Chemistry.  —  Chemistry  treats  of  the  atoms, 
of   the   atomic   structure   of   the   molecules,  of   the 
action    of   affinity,  and   of   chemical   properties  and 
changes. 

39.  Natural    Philosophy.  —  Natural    Philosophy 
includes  mechanics  and  physics. 

QUESTIONS. 

i.  What  do  we  mean  by  a  phenomenon?  2.  Give  illustra- 
tions. 3.  Of  what  do  the  physical  sciences  treat?  4.  Name 
the  principal  physical  sciences.  5.  What  do  we  mean  by  a 
material  unit?  6.  Name  the  three  orders  of  material  units. 
7.  Of  what  does  mechanics  treat  ?  8.  Of  what  does  astronomy 
treat?  9.  What  do  we  mean  by  chemical  properties?  10. 
By  physical  properties?  11.  What  do  we  mean  by  physical 
changes?  12.  By  chemical  changes  ?  13.  Give  illustrations  of 
each.  14.  Of  what  does  physics  treat?  15.  Of  what  does 
chemistry  treat?  16.  What  sciences  are  included  in  natural 
philosophy  ? 


NATURAL  PHILOSOPHY.  23 

CHAPTER    V. 
FIRST   LAW   OF  MOTION. 

40.  Velocity.  —  By    velocity    we    mean    rate    of 
motion.     Velocity  is  usually  stated  in  feet  or  miles 
per  second,  or  in  miles  per  hour. 

Illustrations.  —  When  we  say  a  stone  is  falling  with  a 
velocity  of  twenty  feet  a  second,  we  mean  that  it  is  moving 
fast  enough  to  go  twenty  feet  in  a  second.  When  we  say  that 
a  railway  train  has  a  velocity  of  thirty  miles  an  hour,  we  mean 
that  the  train  is  going  fast  enough  to  go  thirty  miles  in  an 
hour.  When  we  say  that  the  earth  is  moving  in  its  orbit  with 
a  velocity  of  eighteen  miles  a  second,  we  mean  that  the  earth 
is  moving  fast  enough  to  go  eighteen  miles  in  a  second. 

When  we  say  a  body  is  falling  with  a  velocity  of  twenty  feet 
a  second,  we  mean  that  the  body  at  that  particular  instant  is 
moving  fast  enough  to  go  twenty  feet  in  a  second.  It  does 
not  follow  that  the  body  will  actually  go  just  twenty  feet  the 
next  second.  The  body  may  be  stopped  before  the  end  of  the 
second  ;  or,  if  it  is  not  stopped,  it  will  go  faster  and  faster,  and 
so  will  go  more  than  twenty  feet.  When  we  say  that  a  railway 
train  has  a  velocity  of  thirty  miles  an  hour,  we  mean,  that,  at 
the  particular  instant  of  which  we  speak,  the  train  is  going  fast 
enough  to  go  thirty  miles  in  an  hour.  The  train  may  go  faster 
or  slower  a  part  of  the  following  hour,  and  so  actually  go  more 
or  less  than  thirty  miles. 

41.  First  Law  of  Motion.  —  A  body  at  rest  tends 
to  remain  at  rest,  and  a  body  in  motion  tends  to  keep 
moving  in  the  same  direction  and  at  the  same  rate ; 
that   is  to  say,   a  force  must  act   upon   a   body  to 
put  it  in  motion,  or  to  stop  a  body  when  in  motion, 
or  to  change  its  rate  or  direction  of  motion. 

Unless  acted  upon  by  external  forces,  a  moving 


24  NATURAL  PHILOSOPHY. 

body  would  always  go  on  in  a  straight  line  and  at 
a  uniform  rate.  This  seems  to  be  contradicted  by 
common  experience.  All  moving  bodies  at  the  sur- 
face of  the  earth  show  a  decided  tendency  to  stop. 
But  all  such  bodies  are  acted  upon  by  some  external 
force  acting  as  a  resistance.  The  chief  resistances 
to  moving  bodies  are  friction  and  resistance  of  the 
atmosphere. 

Illustrations.  —  A  railway  train  is  stopped  by  the  friction 
of  the  brakes  upon  the  wheels,  and  of  the  wheels  upon  the 
track,  and  by  the  resistance  of  the  air.  A  stone  thrown 
upward  is  stopped  by  the  resistance  of  the  air  and  by  the 
downward  pull  of  gravity. 

In  proportion  as  the  resistances  are  diminished, 
the  longer  will  a  body  continue  in  motion  :  hence 
we  may  reasonably  infer,  that,  were  the  resistances 
entirely  removed,  the  body  would  keep  moving  on 
forever. 

Illustrations.  —  A  smooth  stone  is  soon  brought  to  rest 
when  sliding  over  the  surface  of  the  earth.  The  same  stone 
will  slide  much  longer  over  ice,  where  there  is  less  friction. 
A  top  that  will  spin  for  ten  minutes  in  the  air  will  spin  more 
than  half  an  hour  in  a  vacuum. 

42.  Inertia.  —  The  tendency  of  a  body  to  keep 
at  rest  or  in  motion  is  called  inertia.  The  greater 
the  mass  of  a  body,  the  greater  its  inertia.  In  order 
to  start  a  body,  or  to  stop  it,  or  to  change  its  rate 
or  direction  of  motion,  it  is  necessary  to  overcome 
its  inertia. 

Illustrations.  —  In  jumping  from  a  carriage  or  car  in  motion, 
one  is  liable  to  be  thrown  down,  owing  to  the  tendency  of  the 
body  to  keep  moving  forward  after  the  feet  have  touched 
the  ground. 


NATURAL  PHILOSOPHY, 


It  takes  time  for  a  force  to  overcome  the  inertia 
of  matter :  hence,  when  a  body  receives  a  sudden 
blow,  the  part  of  the  body  immediately  acted  upon 
yields  before  there  is  time  to  overcome  the  inertia 
of  the  surrounding  parts. 

Illustrations. —  If  a  number  of  checkers  are  piled  up  in  a 
column,  one  of  them  may  be  knocked  out  by  a  very  quick  blow 
with  a  table-knife,  without  overturning  the  column.  A  feeble 
blow  will  fail.  Stick  two  needles  into  the  ends  of  a  broom- 
stick, and  rest  the  needles  on  two  glass  goblets,  as  shown  in 
Figure  2.  Strike  the  middle  of  the  stick  a  quick,  sharp  blow 


Fig.  a. 

with  a  heavy  poker.  The  stick  will  break  without  breaking 
the  needles  or  the  goblets.  Here,  again,  a  feeble  or  indecisive 
blow  will  fail.  A  soft  body  fired  fast  enough  will  hit  as  hard 
as  lead.  A  tallow  candle  may  be  fired  from  a  gun  through 
a  pine  board, 

43.  Centrifugal  Force.  —  The  so-called  centrifugal 
force  is  simply  the  tendency  of  the  parts  of  a  rotat- 
ing body  to  keep  moving  in  straight  lines.  This  ten- 
dency increases  with  the  speed  of  rotation,  and 
sometimes  to  such  a  degree  as  to  overcome  the 
cohesion  of  the  body.  In  this  case  the  body  will 
fly  in  pieces,  as  large  grindstones  and  heavy  fly- 
wheels have  been  known  to  do. 


26 


NATURAL  PHILOSOPHY. 


Illustrations.  —  If  a  stone  is  fastened  to  the  end  of  a  string, 
and  twirled  rapidly  around  the  finger,  the  tendency  of  the 
stone  to  fly  off  in  a  straight  line  may  become  sufficient  to 
break  the  string. 

This  tendency  to  move  on  in  a  straight  line  must 
be  counteracted  by  the  force  acting  towards  the 


Fig.  3. 

centre,  in  order  to  keep  a  body  moving  in  a  circle. 
The  faster  a  body  moves,  the  greater  the  pull 
needed  to  keep  the  body  in  its  circular  path.  The 
greater  the  pull  upon  the  body  towards  the  centre, 
the  greater  the  pull  of  the  body  away  from  the 
centre.  The  pull  upon  the  body  towards  the  centre 
is  called  the  centripetal  force,  and  the  pull  of  the 


NATURAL  PHILOSOPHY. 


body  away  from  the  centre  is  called  the  centrifugal 
force. 

Illustrations.  —  The  pull  of  a  revolving  body  away  from  the 
centre  may  be  illustrated  by  the  pieces  of  apparatus  shown  in 
Figures  3  and  4.  In  the 
first,  two  balls,  M  and  M', 
are  placed  on  the  rod  A  B, 
which  passes  through  them. 
The  rod  is  then  put  in  rapid 
rotation  by  turning  the  crank, 
and  the  balls  fly  apart. 

The  flexible  rings  in  Fig- 
ure 4  are  fastened  at  the 
bottom  to  the  upright  shaft, 
but  are  free  to  slide  up  and 
down  upon  it  at  the  top. 
If  these  rings  are  whirled 
in  place  of  the  rod,  they 
will  become  more  and  more 
flattened  as  the  speed  in- 
creases. This  change  of 
form  is  due  to  the  pull  of 
each  part  of  the  rings  away 
from  the  axis.  The  pull 
will  be  greatest  at  the  mid-  Fig-  4- 

die  of  the  rings,  because  this  part  is  moving  fastest.  It  was 
in  this  way  that  the  earth  became  flattened  at  the  poles  while 
in  the  fluid  state. 


QUESTIONS. 

I.  What  is  meant  by  velocity  ?  2,  Give  illustrations.  3.  If 
the  velocity  of  a  body  is  thirty  feet  a  second,  does  it  follow 
that  the  body  will  go  thirty  feet  the  next  second?  4.  Why 
not?  5.  State  the  first  law  of  motion.  6.  What  is  needed  to 
start  a  body,  to  stop  a  body,  or  to  change  its  rate  or  direction 
of  motion?  7.  What  do  all  moving  bodies  at  the  surface  of 


28  NATURAL  PHILOSOPHY. 

the  earth  show ?  8.  Why?  9.  What  are  the  chief  resistances 
encountered  by  moving  bodies  ?  10.  Give  illustrations  of  bodies 
stopped  by  resistance.  11.  What  is  true  of  the  motion  of 
bodies  in  proportion  as  the  resistances  to  their  motions  are 
removed?  12.  What  may  we  rightly  infer  from  this?  13.  Give 
illustrations.  14.  What  do  we  mean  by  the  inertia  of  matter? 
15.  To  what  is  the  inertia  of  a  body  proportional?  16.  It  is 
necessary  to  overcome  its  inertia  in  order  to  do  what  to  a 
body?  17.  Give  illustrations  of  inertia.  18.  What  is  the  effect 
of  a  sudden  blow  upon  a  body?  19.  Why?  20.  Give  illustra- 
tions. 21.  What  do  we  mean  by  centrifugal  force?  22.  Give 
illustrations.  23,  What  is  needed  to  keep  a  body  moving  in 
a  circle  ?  24.  What  name  do  we  give  to  the  pull  towards 
the  centre  ?  25.  What  effect  has  an  increase  of  velocity  upon 
the  centrifugal  force?  26.  Give  the  illustration  of  the  balls. 
27.  Of  the  rings. 


CHAPTER    VI. 

SECOND   LAW   OF   MOTION. 

44.  Impulse.  —  The  effect  of  a  force  in  imparting 
motion  increases  with  the  intensity  of  the  force  and 
with  the  time  during  which  it  acts.  The  product  of 
the  intensity  of  the  force  and  the  time  during  which 
it  acts  is  called  the  impulse  of  the  force. 

EXAMPLE.  —  The  impulse  of  'a  force  of  fifty  poundals  acting 
twelve  seconds  is  equal  to  twelve  times  fifty,  or  six  hundred. 

It  takes  the  same  impulse  to  stop  a  body  that 
it  does  to  put  it  in  motion. 

Illustration.  —  The  greater  the  force  with  which  a  ball  is 
thrown,  the  faster  it  moves,  and  the  harder  it  is  for  the  catcher 
to  stop  it.  It  requires  the  same  exertion  to  stop  the  ball  as 
it  did  to  throw  it. 


NATURAL  PHILOSOPHY.  29 

45.  Momentum. — The  impulse  needed  to  set  a 
body  in  motion  increases  with  the  mass  of  the  body 
and  with  the  velocity  which  is  imparted  to  it.     So, 
also,  the  impulse  needed  to  stop  a  body  increases 
with  the  mass  of   the  body  and  with  its  velocity. 
The  product  of  the  mass  of  a  body  by  its  velocity  is 
called  ^the  momentum  of  the  body. 

EXAMPLE.  —  The  momentum  of  a  mass"  of  eighty  pounds, 
having  a  velocity  of  ninety  feet  a  second,  is  equal  to  ninety 
times  eighty,  or  seven  hundred  and  twenty. 

Illustrations  of  Momentum.  —  If  a  cannon-ball  and  a  mar- 
ble are  struck  equally  hard  with  a  mallet,  the  marble  will  be 
shot  forward  with  a  much  greater  velocity  than  the  cannon-ball, 
but  with  the  same  momentum.  It  would  be  just  as  hard  to 
stop  the  cannon-ball  as  the  marble.  A  large  ship,  even  though 
moving  slowly,  strikes  the  wharf  with  crushing  power,  owing 
to  its  great  momentum.  A  bullet,  though  its  mass  is  small, 
strikes  with  deadly  effect  when  fired  from  a  rifle,  because  its 
great  velocity  gives  it  a  great  momentum.  A  ship  caught 
between  two  icebergs,  whose  motion  is  barely  perceptible,  is 
crushed  as  if  it  were  an  egg-shell,  the  great  mass  of  the  ice- 
bergs giving  them  an  enormous  momentum. 

46.  Second   Law   of    Motion.  —  The  change   of 
momentum  of  a  body  is  equal  to  the  impulse  which 
produces  it,  and  is  in  the  direction  in  which  the  force 
acts. 

EXAMPLE.  —  The  change  of  momentum  produced  by  a  force 
of  eighty  poundals  acting  five  seconds  would  be  equal  to  five 
times  eighty,  or  four  hundred. 

The  velocity  which  a  force  will  impart  to  a  body 
is  directly  proportional  to  the  impulse  of  the  force, 
and  inversely  proportional  to  the  mass  of  the  body ; 
that  is  to  say,  the  velocity  imparted  by  a  force  is 
equal  to  the  quotient  of  the  impulse  divided  by  the 
mass. 


30  NATURAL  PHILOSOPHY. 

EXAMPLE.  —  A  force  of  twelve  poundals  would  impart  in 
twenty  seconds,  to  a  mass  of  five  pounds,  a  velocity  of  240  -f  5, 
or  48  feet  a  second. 

A  force  has  the  same  effect  in  producing  motion, 
whether  it  acts  upon  a  body  at  rest  or  in  motion. 

Illustrations.  —  A  ball  dropped  from  the  ceiling  of  a  car  in 
motion  would  strike  the  floor  in  the  same  place  and  with  the 
same  velocity  as  if  the  car  were  at  rest.  The  ball  while  falling 
has  kept,  also,  the  forward  motion  of  the  car.  If  a  boy  while 
walking  throws  a  ball  straight  up  into  the  air,  he  can  catch  i( 
when  it  comes  down,  just  as  if  he  were  standing  still. 

A  force  has  the  same  effect  in  producing  motion, 
whether  it  acts  alone  or  with  other  forces. 

Illustration.  —  A  steamer  headed  directly  across  a  river  in 
which  there  is  a  swift  current  will  cross  the  river  in  the  same 
time  as  if  there  were  no  current.  It  will  also  be  carried  down 
stream  just  as  far  as  it  would  have  been,  had  it  been  allowed 
to  float  quietly  with  the  current.  The  steam  and  the  current 
each  carries  the  boat  just  as  far  in  the  direction  in  which  each 
acts  as  if  acting  alone. 

47.  Parallelogram  of  Motion.  —  To  find  the  path 
of  a  body  A  (Figure  5)  acted  on  by  two  forces  at 
the  same  time,  draw  A  B  to  represent 
the  path  the  body  would  have  taken, 
had  it  been  acted  on  by  the  first  force 
alone,  and  A  C  to  represent  the  path 
it  would  have  taken,  had  it  been  acted 
on  by  the  other  force  alone.  Through 
B  draw  BD  parallel  to  AC,  and 


through  C  draw  CD  parallel  to  AB, 
so  as  to  complete  the  parallelogram  AB DC.  Draw 
the  diagonal  A  D.  This  diagonal  will  represent  the 
path  taken  by  the  body  when  acted  upon  by  both 
fprces  together. 


NATURAL  PHILOSOPHY.  31 

48.  Falling  Bodies.  —  We  have  illustrations  of 
the  second  law  of  motion  in  the  case  of  falling 
bodies.  Were  it  not  for  the  resistance  of  the  air, 
gravity  would  increase  the  velocity  of  a  falling  body 
at  the  rate  of  about  thirty-two  feet  a  second.  Start- 
ing from  a  state  of  rest,  gravity  would  give  a  body 
a  velocity  of  thirty-two  feet  during  the  first  second. 
During  the  next  second  gravity  will  have  the  same 
effect  upon  the  body  as  during  the  first  second, 
although  the  body  is  already  in  motion,  and  will  give 
it  an  additional  velocity  of  thirty-two  feet ;  and  so 
for  each  succeeding  second. 

To  find  the  velocity  of  a  body  falling  from  a  state 
of  rest,  multiply  thirty-two  by  the  number  of  seconds 
it  has  been  falling. 

EXAMPLE.  —  The  velocity  of  a  body  falling  from  a  state  of 
rest,  were  it  not  for  the  resistance  of  the  air,  would  be,  at  the 
end  of  the  eighth  second,  eight  times  thirty-two,  or  two  hun- 
dred and  fifty-six  feet. 

Gravity  causes  a  body  falling  from  a  state  of  rest 
to  fall  sixteen  feet  the  first  second  ;  and  during  each 
subsequent  second  it  causes  it  to  fall  sixteen  feet 
more  than  its  velocity  at  the  beginning  of  the. 
second. 

Illustration.  —  At  the  beginning  of  the  second  second,  the 
velocity  of  a  body  falling  from  a  state  of  rest  is  thirty-two  feet. 
This  velocity  would  of  itself  carry  the  body  down  thirty-two 
feet  during  this  second.  Gravity  causes  it  to  fall  sixteen  feet 
farther:  hence  it  would  fall  32  +  16,  or  48  feet,  the  second 
second.  During  the  third  second  it  will  fall  64  +  16,  or  80 
feet;  during  the  fourth  second  it  will  fall  96+16,  or  112  feet, 
etc.  It  appears,  then,  that  a  body  will  fall  three  times  as  far 
the  second  second  as  the  first,  five  times  as  far  the  third, 


32  NATURAL  PHILOSOPHY. 

seven  times  as  far  the  fourth,  etc.  The  distance  each  second 
increases  in  the  ratio  of  the  odd  numbers,  three,  five,  seven, 
nine,  etc. 

49.  Rising  Bodies.  —  Gravity  changes  the  velocity 
of  a  rising  body  thirty-two  feet  a  second,  but  in  this 
case  it  retards  the  body. 

To  find  the  velocity  of  a  rising  body  at. any  time, 
multiply  thirty-two  by  the  number  of  seconds  it  has 
been  rising^  and  deduct  the  product  from  the  velocity 
with  which  it  started, 

EXAMPLE.  —  A  body  starts  upward  with  a  velocity  of  nine 
hundred  feet  a  second.  What  would  be  its  velocity  at  the  end 
of  the  twentieth  second  ?  32  x  20  =  640.  900  —  640  =  260. 
The  velocity  would  be  two  hundred  and  sixty  feet. 

Gravity  would  cause  a  rising  body  to  rise  each 
second  sixteen  feet  less  than  its  velocity  at  the 
beginning  of  the  second. 

EXAMPLE.  —  If  a  body  started  upwards  with  a  velocity  of 
five  hundred  feet,  it  would  rise  500  —  16,  or  484  feet.  Its 
velocity  at  the  end-  of  this  second  will  be  500  —  32,  or  468  feet. 
The  next  second  it  will  rise  468  — 16,  or  452  feet.  The  third 
second  its  velocity  will  be  468—32.  or  436  feet;  and  it  will  rise 
this  second  436  —  16,  or  420  feet:  and  so  on. 

50.  A   Body  Projected   Horizontally.  —  Gravity 
would  have  the  same  effect  upon  a  ball  fired  hori- 
zontally as  upon  one  dropped  from  the  same  height. 
Each  would  strike  the  ground  at  the  same  time. 

Illustration.  —  Suppose  a  ball  fired  horizontally  in  the  direc- 
tion ^/''(Figure  6),  with  a  velocity  which  would  carry  it  from 
A  to  F  in  five  seconds.  Divide  the  line  A  F  into  five  equal 
parts,  AB,  BC,  CD,  D  E,  and  EF.  Let  the  vertical  line 
A  L  represent  the  distance  a  body  would  fall  from  A  in  five 
seconds.  Let  A  G  represent  the  distance  the  body  would  fall 


NATURAL   PHILOSOPHY. 


33 


the  first  second,  GH  the  distance  it  would  fall  the  next  second, 
HI  the  distance  it  would  fall  the  third  second,  IK  the  distance 
it  would  fall  the  fourth  second,  and  K L  the  distance  it  would 
fall  the  fifth  second. 

During   the    first   second    the    body   projected    horizontally 
would  move  forward  as  far  as  from  A  to  B,  and  would  be 
pulled  down  by  gravity  from  B  to  r,  a  distance  equal  to  A  G. 
A  B  c  D  i  » 


Fig.  6. 

At  the  end  of  the  next  second  the  ball  will  have  moved  for- 
ward to  C,  and  have  been  pulled  down  to  2,  a  distance  equal 
to  A  H,  etc.  Hence,  at  the  end  of  the  first  second,  the  ball  will 
be  at  i,  at  the  end  of  the  second  second  at  2,  at  the  end  of 
the  third  second  at  3,  at  the  end  of  the  fourth  second  at  4,  and 
at  the  end  of  the  fifth  second  at  5.  The  ball  would  actually 
have  moved  on  the  curved  path  from  A  to  5.  This  curve  is 
called  a  parabola. 

QUESTIONS. 

r.  With  what  does  the  effect  of  a  force  in  imparting  motion 
increase?  2.  What  clo  we  mean  by  the  impulse  of  a  force? 
3.  Give  an  example.  4.  What  is  true  of  the  impulse  required 
to  start  and  to  stop  a  body?  5.  Give  an  illustration.  6.  With 
what  does  the  impulse  needed  to  put  a  body  in  motion  increase  ? 


34  NATURAL  PHILOSOPHY. 

7.  What  do  we  mean  by  the  momentum  of  a  body  ?  8.  Give 
an  example.  9.  Give  illustrations  of  momentum.  10.  State 
the  second  law  of  motion,  n.  Give  an  example.  12.  What 
is  true  of  the  velocity  which  a  force  will  impart  to  a  body? 
13.  Give  an  example.  14.  What  is  true  of  the  effect  of  a  force 
in  producing  motion,  whether  it  act  upon  a  body  at  rest  or 
in  motion?  15.  Give  an  illustration.  16.  What  is  true  of  the 
effect  of  a  force  in  producing  motion,  whether  it  act  alone  or 
with  other  forces  ?  17.  Give  an  illustration.  18.  Describe  the 
parallelogram  of  motion.  19.  Falling  bodies  are  illustrative  of 
which  law  of  motion  ?  20.  Were  it  not  for  the  resistance  of 
the  atmosphere,  gravity  would  increase  the  velocity  of  a  falling 
body  how  much  each  second?  21.  How  may  we  find  the 
velocity  of  a  body  at  the  end  of  any  given  second  ?  22.  Give 
an  example.  23.  How  far  does  gravity  cause  a  body  to  fall 
each  second?  24.  Give  an  illustration.  25.  How  much  does 
gravity  change  the  velocity  of  a  rising  body  each  second? 
26.  Give  an  example.  27.  What  effect  has  gravity  upon  the 
distance  a  body  rises  each  second?  28.  Give  an  example. 

29.  What  effect   has   gravity  upon  a  body  fired  horizontally? 

30.  Give  an  illustration. 


CHAPTER    VII. 

THIRD    LAW   OF   MOTION. 

51.  Third  Law  of  Motion.  —  Reaction  is  always 
eqtial  and  opposite  to  action ;  that  is  to  say,  the 
actions  of  two  bodies  upon  each  other  are  always 
equal,  and  in  opposite  directions. 

This  law  simply  states  the  fact  that  a  force  always 
acts  upon  two  portions  of  matter,  and  that  the  stress 
is  equal  upon  both  portions. 

Illustrations.  —  A  stone  raised  from  the  earth  attracts  the 
earth  just  as  much  as  the  earth  attracts  the  stone.  Gravity 


NATURAL   PHILOSOPHY. 


35 


pulls  them  equally,  but  in  opposite  directions.  When  the 
stone  falls,  the  earth  moves  up  to  meet  it.  When  the  two 
meet,  they  have  each  the  same  momentum  ;  but  the  earth, 
owing  to  its  great  mass,  has  only  a  very  small  velocity.  When 
a  cannon  is  fired,  the  powder  pushes  back  upon  the  cannon 
just  as  hard  as  it  pushes  forward  on  the  ball..  Were  the 
cannon  as  free  to  move  as  the  ball,  it  would  start  back,  or 
recoil^  with  the  same  momentum  that  the  ball  starts  forward 
with,  but  of  course  with  a  less  velocity. 

We  have  an  illustration  of  action  and  reaction  in  the  col- 
lision of  elastic  bodies.  Place  two  ivory  balls  of  exactly  the 
same  size  at  the 
centre  of  the  curved 
railway  in  Figure  7. 
Move  one  of  the 
balls  up  to  one  end 
of  the  track,  and 
let  it  roll  back 
against  the  ball  at 
rest.  There  will  be 
a  slight  strain  of 
compression  when 
the  balls  strike,  and 
this  will  develop  a 
stress  of  elasticity  between  them,  which  will  act  equally  upon 
both,  and  in  opposite  directions.  This  stress  will  stop  the  first 
ball,  and  start  the  second  off  with  the  velocity  the  first  had 
on  striking  it. 

Place  several  ivory  balls  of  the  same  size  on  the  centre  of 
the  track,  and  allow  the  first  ball  to  roll  against  the  end  of 
the  line.  All  the  balls  will  remain  at  rest  except  the  last, 
which  will  be  shot  up  the  track.  In  this  case  the  strain  of 
compression  and  stress  of  elasticity  have  been  sent  along  the 
line  from  ball  to  ball.  Each  ball  has  been  compressed  a  little 
in  turn,  and  in  recovering  itself  has  pushed  upon  the  ball 
behind  it  enough  to  stop  it,  and  upon  the  o.ne  in  front  enough 
to  flatten  it  a  little.  Each  ball  except  the  last  was  kept  from 
moving  forward  by  the  reaction  of  the  ball  in  front. 


nnnnfino 


Fig.  7. 


NATURAL   PHILOSOPHY. 


52.  Reflected  Motion.  —  When  an  elastic  body 
is  thrown  against  a  hard,  smooth  surface,  reaction 
causes  it  to  rebound.  '  If  it  is  thrown  in  a  direction 

perpendicular  to  the  sur- 
face, it  will  rebound  in  the 
same  direction  ;  if  thrown 
obliquely,  it  will,  rebound 
obliquely  in  an  opposite 
direction.  The  direction 
in  which  the  body  ap- 
proaches the  reflecting 
surface  is  its  line  of  inci- 
dence, and  that  in  which  it 
rebounds  the  line  of  reflec- 
tion. The  angle  Included 
between  the  line  of  inci- 
dence and  a  perpendicular 
to  the  surface  is  called 
the  angle  of  incidence. 
The  angle  included  between  the  line  of  reflection 
and  the  perpendicular  is  called  the  angle  of  reflec- 
tion. 


Fig.  8. 


Fig.  9. 

The  angle  of  reflection  is   equal  to   the  angle  of 
incidence.      This  is  the  law  of  reflected  motion. 


NATURAL  PHILOSOPHY.  37 

Illustrations. —  If  an  ivory  ball  is  allowed  to  fall  upon  a 
marble  slab,  as  shown  in  Figure  8,  it  will  rebound  perpendicu- 
larly nearly  to  the  height  from  which  it  fell.  If  the  ball  were 
shot  from  A,  Figure  9,  it  would,  on  striking  the  surface  at  B, 
rebound  at  C,  making  the  angle  of  reflection,  CBD,  equal  to 
the  angle  of  incidence,  A  BD. 

QUESTIONS. 

i.  State  the  third  law  of  motion.  2.  What  fact  does  this 
law  state  ?  3.  Give  the  illustration  of  the  falling  stone.  4.  Of 
the  cannon  and  ball.  5.  Of  two  elastic  balls.  6.  Of  several 
elastic  balls.  7.  What  takes  place  in  reflected  motion?  8. 
What  is  meant  by  the  line  of  incidence?  9.  By  the  line  of 
reflection?  10.  By  the  angle  of  incidence?  n.  By  the  angle 
of  reflection?  12.  What  is  the  law  of  reflected  motion?  13. 
Give  illustration. 


CHAPTER    VIII. 

CENTRE    OF   GRAVITY   AND   EQUILIBRIUM. 

53.  Centre  of  Gravity.  —  There  is  for  every  body 
a  point  on  which  it  will  balance  in  every  position 
in  which  it  can  be  placed.  This  point  is  called  the 
centre  of  gravity  of  the  body.  It  received  this  name 
because  the  force  of  gravity,  acting  on  the  body 
on  one  side  of  this  point,  is  always  balanced  by 
that  acting  on  the  other  side. 

The  centre  of  gravity  of  a  body  always  seeks  to 
get  into  the  lowest  possible  position.  When  a  body 
at  rest  is  suspended  by  a  string,  its  centre  of 
gravity  will  always  be  in  a  vertical  line  under  the 
point  of  support.  The  force  of  gravity,  which  is 
really  acting  upon  all  the  particles  of  which  a  body 


38  NATURAL  PHILOSOPHY. 

is  composed,  has  the  same  effect  upon  the  body,  as 
a  whole,  as  if  it  were  all  applied  to  its  centre 
of  gravity. 

54.  Position  of  the  Centre  of  Gravity.  —  When 
a  body  is  uniform  throughout,  its  centre  of  gravity 
is  at  its  centre  of  figure. 

Illustrations.  —  The  centre  of  gravity  of  a  circle  or  other 
regular  figure  cut  out  of  a  board  is  at  the  centre  of  the  figure  ; 
and  the  centre  of  gravity  of  a  sphere  is  at  the  centre  of  the 
sphere. 

When  the  body  is  not  uniform  throughout,  its 
centre  of  gravity  will.be  away  from  its  centre  of 
figure,  toward  the  denser  or  heavier  side  of  the 
body. 

Illustrations.  —  The  centre  of  gravity  of  dice  is  at  their 
centre  of  figure,  and  they  are  as  liable  to  fall  with  one  side 
down  as  another  when  thrown;  but  the  centre  of  gravity  of 
loaded  dice  is  nearer  their  loaded  side,  so  that  they  are  very 
apt  to  fall  with  that  side  down. 

The  centre  of  gravity  often  lies  entirely  outside 
of    the   material   of    the    body.     When    this    is    the 
^         c         ^   case,  the  centre  of  gravity  must  be 
^        z         ^   rigidly    connected    with    the    body 
in  order  to  have  the  body  balance 
on  it. 

Illustrations.  —  The  centre   of  gravity 
Fig-  I0-  of  a  ring  lies  in  the   space  enclosed  by 

the  ving,  and  the  centre  of  gravity  of  a  tin  pail  lies  within  the 
space  enclosed  by  the  tin. 

A  system  of  bodies  may  have  a  common  centre 
of  gravity  lying  outside  of  all  of  the  bodies. 


NATURAL  PHILOSOPHY, 


39 


Illustrations.  —  The  centre  of  gravity  of  two  spheres  (Fig- 
ure 10)  will  lie  somewhere  on  a  line  between  their  centres  of 
gravity.  If  the  spheres  have  the  same  mass,  this  point  will 
be  just  midway  between 
these  centres.  If  one 
sphere  has  a  greater  mass 
than  the  other,  the  centre 
of  gravity  of  the  system 
will  be  nearer  the  centre 
of  gravity  of  the  larger 
sphere.  If  there  is  suffi- 
cient difference  between 
their  masses,  their  com- 
mon centre  of  gravity  may 
lie  within  the  larger 
sphere. 

55.  Kinds  of  Equi- 
librium.—  A  body  at 
rest  is  said  to  be  in 
equilibrium.  When  a 
body,  on  being  tipped 
a  little,  tends  to  return 
to  its  old  position,  it  is  said  to  be  in  stable  equilib- 
rium ;  when  it  tends  to  fall  to  a  new  position,  in 
unstable  equilibrium ;  and,  when  it  rests 
equally  well  in  every  position,  in  indif- 
ferent equilibrium. 

When  a  body  is  in  stable  equilibrium, 
its  centre  of  gravity  rises  on  tipping 
the  body ;  when  it  is  in  unstable  equi- 
librium, its  centre  of  gravity  falls  on 
tipping  the  body  ;  and,  when  it  is  in 
indifferent  equilibrium,  its  centre  of  gravity  neither 
rises  nor  falls  on  tipping  the  body. 


Fig.   it. 


Fig.  12. 


NATURAL   PHILOSOPHY. 


Illustrations.  —  A  rod  balanced  on  the  finger,  as  shown  in 
Figure  11,  is  in  unstable  equilibrium,  the  centre  of  gravity 
being  above  the  point  of  support. 
As  soon  as  the  rod  begins  to  tip, 
its  centre  of  gravity  begins  to  fall. 
A  cork  balanced  on  the  point  of  a 
needle,  as  shown  in  Figure  12,  is 
in  stable  equilibrium,  as  the  heavy 
forks  bring  the  centre  of  gravity 
below  the  point  of  support.  As 
soon  as  the  cork  begins  to  tip,  its 
centre  of  gravity  begins  to  rise.  A 
sphere  on  a  level  surface  is  in  in- 
different equilibrium.  The  sphere 
rests  on  a  single  point ;  but,  when 
the  body  is  tipped,  the  centre  of 
gravity  always  remains  at  the  same 
distance  above  the  point.  A  man 
.  13.  on  stilts,  as  shown  in  Figure  13,  is 

in  stable  equilibrium  in  one  direction,  and  in  unstable  equilib- 
rium in  another,  the  centre  of 
gravity  being  above  the  points 
of  support.  The  man  is  very 
liable  to  fall  forward  or  back- 
ward, but  not  likely  to  fall  to 
the  right  or  left.  As  soon  as 
he  leans  forward  or  backward, 
his  centre  of  gravity  begins  to 
fall ;  but  when  he  leans  to  the 
right  or  left,  at  first  his  centre 
of  gravity  rises.  A  table  stand- 
ing on  three  legs,  as  shown  in 
Figure  14,  is  in  stable  equilib- 
rium in  every  direction.  The 
table  cannot  be  tipped  without  raising 


Fig.  14.      ' 

its  centre  of  gravity. 


The   lower  the   centre   of  gravity  of  a  body,  and 
the   broader  its   base,  the  greater  its  stability.     A 


NATURAL   PHILOSOPHY.  4! 

body  will  stand  firmly,  even  though  it  leans,  pro- 
vided a  vertical  line  from  its  centre  of  gravity  falls 
within  its  base. 

Illustrations.  —  A  loaded  wagon  on  an  uneven  road  may  tip 
considerably  to  one  side  without  being  overturned.  If  the 
load  is  piled  up  high,  it  is  in  greater  danger  of  being  over- 
turned, as  the  vertical  line  from  the  centre  of  gravity  is  more 
likely  to  fall  outside  the  base  when  the  wagon  tips.  The 
famous  Leaning  Tower  of  Pisa,  though  it  looks  very  unstable, 
is  in  stable  equilibrium,  as  a  vertical  line  from  its  centre  of 
gravity  falls  within  the  base. 

QUESTIONS. 

i.  What  is  meant  by  the  centre  of  gravity  ?  2.  Why  was  it 
so  named  ?  3.  What  position  does  it  always  seek  ?  4.  When 
a  body  at  rest  is  hung  by  a  string,  where  will  its  centre  of 
gravity  be?  5.  What  would  be  the  effect  of  gravity  upon  a 
body  as  a  whole,  were  it.  all  applied  to  the  centre  of  gravity  ? 

6.  When    is    the    centre    of   gravity   at   the    centre   of   figure? 

7.  Give  illustrations.     8.  When  is  the  centre  of  gravity  away 
from  the  centre  of  figure  ?     9.  Give  illustrations.     10.  Is  the 
centre   of   gravity  always   in    the    material    of  the   body?     n. 
Give  some  cases  where  it  is  not.     12.  What  does  any  system 
of  bodies  have?     13.  Give  illustrations.     14.  When  is  a  body 
said  to  be  in  equilibrium?     15.  Name  the  three  kinds  of  equi- 
librium.    1 6.  How  can  you  tell  in  which  equilibrium  any  body 
is?     17.  What  happens  to  the  centre  of  gravity  of  a  body  on 
tipping  it  in  each  case  of  equilibrium?     18.  Give  the  illustra- 
tion of  the  rod  balanced  on  the  finger.     19.  Of  the  cork  bal- 
anced on  a  needle.     20.  Of  a  sphere  on  a  level  surface.     21 
Of  a  man  on  stilts.     22.  Of  a  table  on  three  legs.     23.  Upon 
what  does  the  stability  of  a  body  depend  ?     24.  To  what  extent 
may  a  body  lean,  and  yet  stand  firmly  ? 


II. 

ENERGY   AND    MACHINES. 


CHAPTER    IX. 

WORK   AND   ENERGY. 

56.  Position  of  Advantage. — A  body  is  said  to 
be   in    a  position  of  advantage  with    respect    to    a 
force,  when  it  is  so  situated  that  it  is  possible  for 
the  force  to  move  it. 

Illustrations.  —  When  a  body  is  raised  from  the  earth,  it  is 
in  a  position  of  advantage  with  respect  to  gravity,  which  may 
then  put  the  body  in  motion  by  pulling  it  to  the  earth.  We 
may  say  that  the  body  may,  by  means  of  gravity,  pull  itself  to 
the  earth.  A  watch-spring,  when  wound  up,  is  in  a  position  of 
advantage  with  reference  to  elasticity,  since  it  is  possible  for 
the  spring  to  put  itself  in  motion  by  unwinding  itself  by  means 
of  its  elasticity. 

57.  Work.  —  Work  is  said  to  be  done  when  any 
portion  of  matter  is  moved  against  resistance.     Work 
may  be  done  not  only  by  men  and  animals,  but  also 
by  forces. 

Illustrations.  —  Work  is  done  when  a  man  raises  a  weight, 
or  when   a   horse  draws  a  load,  or  when  the  wind  drives  a 
vessel,  or  when  gravity  pulls  down  a  clock-weight. 
42 


NATURAL  PHILOSOPHY.  43 

There  are  two  kinds  of  work.  One  is  that  of 
putting  portions  of  matter  into  positions  of  advan- 
tage, and  the  other  is  that  of  quickening  their  speed. 
The  resistance  overcome  in  the  latter  case  is  that 
of  inertia. 

Illustrations.  —  When  a  weight  is  raised  from  the  earth,  the 
work  done  by  the  force  which  raises  the  t weight  is  that  of 
putting  it  in  a  position  of  advantage.  When  the  weight  is 
allowed  to  fall  freely  to  the  earth  again,  the  work  done  by 
gravity  is  that  of  increasing  the  speed  of  the  body  by  over- 
coming its  inertia. 

58.  Measurement  of  Work.  —  The  work  done  in 
any  case  is  equal  to   the  product   of  the  force  em- 
ployed and  the  distance  through  which  it  acts  upon 
the  body.     A  foot-pound  of  work  is  the  work  done 
by  a  force  of  one  pound  acting  through  a  distance 
of  one  foot,  or  the  work  done  in  raising  a  pound  one 
foot  high.     A  foot-poundal  of  work  is  the  work  done 
by  a  poundal  of  force  acting  through  one  foot,  or 
the  work  done  in  raising  half  an  ounce  one  foot  high. 

Illustrations.  —  If  I  wish  to  raise  five  pounds  twelve  feet 
high,  I  must  exert  a  force  of  five  pounds ;  and  that  force  must 
act  upon  the  body  for  a  distance  of  twelve  feet :  the  work 
done  is  five  times  twelve,  or  sixty  foot-pounds.  The  work 
done  by  eight  poundals  of  force  acting  upon  a  body  through 
a  distance  of  twelve  feet  is  equal  to  eight  times  twelve,  or 
ninety-six  foot-poundals. 

59.  Energy.  —  By    energy    is    meant    capacity  for 
doing  work. 

Illustrations.  —  A  weight  raised  from  the  earth  is  able  to 
pull  itself  to  the  earth,  and  therefore  has  the  capacity  for  doing 
a  certain  amount  of  work.  The  work  done  by  the  falling 
weight  may  be  either  that  of  increasing  its  own  velocity,  when 


44  NA  TURAL   PHILOSOPHY. 

it  falls  freely,  or  of  putting  other  bodies  in  motion,  as  when  it 
is  attached  to  the  wheels  of  a  clock. 

A  spring  when  wound  up  has  a  capacity  for  doing  a  cer- 
tain amount  of  work.  In  unwinding,  it  may  increase  its  own 
velocity,  or  it  may  put  in  motion  the  wheels  of  a  watch. 

Running  water  has  a  capacity  for  work.  It  may,  for  instance, 
turn  the  wheels  of  a  mill. 

60.  Two  Kinds  of  Energy.  —  Every    portion    of 
matter  in  a  position  of  advantage  possesses  energy, 
or  capacity  for  doing  work ;    and  the  same  is  true 
of  every  portion  of  matter  in  motion. 

The  energy  of  a  body  due  to  its  position  of 
advantage  is  called  energy  of  position,  or  potential 
energy.  The  energy  of  a  body  due  to  its  motion 
is  called  energy  of  motion,  or  kinetic  energy. 

Illustrations.  —  The  energy  of  a  raised  clock-weight,  or  of 
a  coiled  spring,  is  potential;  while  that  of  running  water,  or 
of  a  ball  fired  from  a  cannon,  is  kinetic. 

The  energy  of  visible  masses  of  matter,  whether 
potential  or  kinetic,  is  called  molar,  or  mechanical 
energy ;  while  the  energy  of  the  molecules  and 
atoms  of  a  body,  whether  potential  or  kinetic,  is 
called  molecular  energy. 

61.  Expenditure  of  Energy.  —  Energy  is  always 
expended  when  work  is  done. 

Illustrations.  —  When  I  raise  a  weight,  I  expend  upon  it  a 
certain  amount  of  the  energy  of  my  arm.  When  the  weight 
falls  to  the  earth  again,  it  is  drawn  to  the  earth  by  the  expendi- 
ture of  its  energy  of  position.  When  work  is  done  by  a  mov- 
ing body,  it  is  always  at  the  expense  of  some  of  its  energy  of 
motion.  The  speed  of  the  body  is  lessened. 

The  energy  expended  by  a  force  in  doing  work 
is  always  equal  to  the  product  of  the  intensity  of 


NATURAL   PHILOSOPHY.  45 

the  force  and  the  distance  through  which  it  acts. 
Energy  is  measured  in  foot-pounds  and  in  foot- 
poundals ;  a  foot-pound  of  energy  being  the  energy 
used  in  doing  a  foot-pound  of  work,  and  a  foot- 
poundal  of  energy  that  used  in  doing  a  foot-poundal 
of  work. 

Illustrations.  —  The  energy 'expended  by  gravity  in  pulling 
five  pounds  down  eight  feet  is  equal  to  eight  times  five  foot- 
pounds, or  forty  foot-pounds.  The  energy  expended  in  raising 
a  weight  of  fifteen  pounds  twenty  feet  is  equal  to  twenty  times 
fifteen  foot-pounds,  or  three  hundred  foot-pounds. 

The  energy  of  a  body  in  a  position  of  advantage 
or  in  motion  is  always  exactly  equal  to  the  energy 
expended  in  putting  a  body  in  its  position  of  advan- 
tage, or  in  giving  the  body  its  motion. 

Illustrations.  —  It  takes  eighty  foot-pounds  of  energy  to 
raise  ten  pounds  eight  feet  from  the  earth  ;  and  ten  pounds 
raised  eight  feet  from  the  earth  possesses  just  eighty  foot- 
pounds of  potential  energy.  It  requires  six  hundred  foot- 
poundals  of  energy  to  give  a  ball  weighing  three  pounds  a 
velocity  of  twenty  feet  a  second  ;  and  a  ball  weighing  three 
pounds,  and  having  a  velocity  of  twenty  feet  a  second,  has  just 
six  hundred  foot-poundals  of  kinetic  energy. 

It  takes  just  as  much-  energy  to  stop  a  body  as 
it  does  to  put  it  in  motion. 

62.  Transformation  of  Energy.  —  When  a  body 
is  thrown  upwards,  its  energy  is  at  first  entirely 
kinetic.  As  it  rises,  it  moves  slower  and  slower, 
and  therefore  loses  kinetic  energy ;  but,  as  it  rises 
higher  and  higher  from  the  earth,  it  gains  potential 
'energy.  At  its  highest  point  the  energy  is  entirely 
potential  When  it  falls,  there  is  a  gradual  loss 


46  NATURAL  PHILOSOPHY. 

of  potential  energy,  since  the  body  comes  nearer 
and  nearer  to  the  earth.  At  the  same  time  there 
is  a  gradual  gain  of  kinetic  energy,  since  the  body 
moves  faster  and  faster.  When  the  body  strikes 
the  earth,  the  motion  of  the  body  as  a  whole  is 
stopped ;  but  its  molecules  and  atoms  are  made  to 
vibrate  with  greater  rapidity.  Its  molar  energy  is 
now  converted  into  molecular  energy. 

While  the  body  was  rising,  kinetic  energy  was 
changed  into  potential  energy ;  while  it  was  falling, 
its  potential  energy  was  again  changed  back  into 
kinetic  energy  ;  and  when  it  struck  the  earth  its 
molar  energy  was  changed  into  molecular  energy. 

Whenever  work  is  done,  one  kind  of  energy  is 
changed  into  another,  — either  kinetic  into  potential, 
or  molar  into  molecular,  or  the  reverse. 

63.  Energy  is  Indestructible.  —  Whatever  trans- 
formations energy  may  undergo,  the  amount  of  energy 
always  remains  the  same.  We  can  no  more  create 
or  destroy  energy  than  we  can  create  or  destroy 
matter. 

QUESTIONS. 

i.  When  is  a  body  said  to  be  in  a  position  of  advantage? 
2.  Give  illustrations.  3.  When 'is  work  said  to  be  done?  4, 
Give  illustrations.  5.  What  are  the  two  kinds  of  work?  6. 
Give  illustrations.  7.  To  what  is  the  work  done  in  any  case 
equal  ?  8.  What  is  a  foot-pound  of  work  ?  9.  A  foot-poundal 
of  work?  10.  Give  the  illustration  of  the  amount  of  work 
done.  ii.  What  is  meant  by  energy?  12.  Give  illustrations. 
13.  What  are  the  two  kinds  of  energy?  14.  Give  illustra- 
tions. 15.  What  is  meant  by  molar  energy?  16.  By  molecu-, 
lar  energy  ?  17.  What  is  always  expended  when  work  is  done  ? 
1 8.  Give  illustrations.  19.  To  what  is  the  energy  expended  by 


NATURAL  PHILOSOPHY.  4? 

a  force  equal?  20.  What  is  meant  by  a  foot-pound  of  energy? 
21.  By  a  foot-poundal  of  energy?  22.  To  what  is  the  energy 
of  a  body  in  a  position  of  advantage  or  in  motion  equal  ?  23. 
Give  illustrations.  24.  How  much  energy  does  it  take  to  stop 
a  body  ?  25.  Give  all  the  transformations  of  energy  which 
take  place  when  a  body  is  thrown  upwards.  26.  What  takes 
place  whenever  work  is  done?  27.  Can  energy  be  destroyed? 


CHAPTER    X, 

MACHINES. 

64.  Definition  of  a  Machine.  —  A  machine  is  an 
instrument  by  which  a  force  may  be  applied  to  do 
work.     The   force  applied   to   a   machine   is   called 
the  power.     The  work  done  by  a  machine  is  that 
of   raising  weight,  or  of   overcoming  some  form  of 
resistance. 

Illustration.  —  A    pair    of    scissors   (Figure    i)  is   a  good 
example    of    a  simple 
machine.     The  power, 
which      is     here     the 
strength     of    the     fin- 
gers, is  applied  to  the 
handles,  so  as  to  cause 
them   to  come  together.     The  work  done  is  that  of  cutting 
some  material  between  the  blades. 

65.  Mechanical   Powers.  —  Air  machines,    how- 

F  y  ever  complicated,  are  construct- 
ed out  of  four  elements,  called 
the  mechanical  powers,  or  the 
Fig.  16.  simple    machines.      These    are 

the  lever,  the  wheel  and  axle,  the  pulley,  and  the 
inclined  plane. 


48 


NATURAL  PHILOSOPHY. 


66.  The  Lever.  —  The  lever  is  a  rigid  bar  capa- 
ble of  turning  upon  a  fixed  point,  or  axis.  The 
point  on  which  the  lever  turns  is  called  the  ful- 
crum ;  and  the  distances  from  the  fulcrum  to  the 


Fig.  17.  Fig.  18. 

points  where  the  power  and  weight  are  applied  are 
called  the  arms  of  the  lever. 

When  the  fulcrum  is  between  the  weight  and 
power,  the  lever  is  said  to  be  of  the  first  order 
(Figure  16) ;  when  the  weight  is  between  the  ful- 


Fig.  19. 


crum  and  power,  the  lever  is  said  to  be  of  the 
second  order  (Figure  17);  and,  when  the  power  is 
between  the  weight  and  the  fulcrum,  the  lever  is 
said  to  be  of  the  third  order  (Figure  18). 


NATURAL  PHILOSOPHY. 


49 


Illustrations.  —  An  iron  bar  used  for  raising  weights  is  a 
lever.  A  pair  of  scissors  (Figure 
15),  a  balance  (Figure  19),  and  a 
pair  of  steelyards  (Figure  20),  are 
levers  of  the  first  order.  A  bar 
placed  under  a  weight,  so  as  to 
raise  it  (Figure  21),  and  a  pair  of 
nut-crackers  (Figure  22),  are  ex- 
amples of  levers  of  the  second 
order.  The  foot-crank  (Figure  23) 


Fig.  20. 


and  the  sugar-tongs  (Figure  24)  are  levers  of  the  third  order. 


Fig.  21.  Fig.  22. 

67.  The  Wheel  and  Axle.  —  'The  wheel  and  axle 

consists,  of  a  wheel 
or  drum,  a  (Figure 
25),  mounted  on  an 
axle,  b.  The  power 
and  weight  are  ap- 
plied to  ropes,  which 
pass,  one  over  the 
wheel  and  the  other 
over  the  axle,  in  op- 
posite directions  ;  so 
that  one  unwinds  as 
the  other  winds  up. 

Illustrations.    •  -    The 
wheel   and    axle    is  often 
used    for  drawing  water; 
the     bucket,     or     weight, 
Fig.  23.  being  applied  to  the  axle, 


NATURAL  PHILOSOPHY. 


and  the   counter-weight,  or  power,  to  the  wheel.     Elevators 
are  usually  raised  by  means  of  a  wheel  and  axle.     In  almost 


Fig.  24.  Fig.  25. 

every  kind  of  mill-work,  wheels  and  axles  are  combined  so  as 
to  act  upon   each  other  by  bands  or  belts  (Figures  26  and 


Fig.  26.  Fig.  27. 

27),  or  by  cogs  (Figures  28  and  29).  The  windlass  (Figure  30) 
and  the  capstan  (Figures  31  and  32)  are  modifications  of  the 
wheel  and  axle. 

68.  The  Pulley.  —  The  pulley  is  a  small  grooved 

wheel  turning  freely  in 
a  frame  called  a  block. 
It  is  a  machine  in  which 
power  is  applied  to  do 
work  by  means  of  a 
cord,  instead  of  a  bar, 
as  in  the  case  of  the 
lever.  The  wheel  of 
the  pulley  serves  simply 
to  diminish  friction  at 
Fig.  28.  the  points  over  which 

the  cord  is  drawn.     When  the  block  is  stationary, 


NATURAL  PHILOSOPHY. 


as  in  the  case  of  the  upper  block,  C9  in  Figure 
33,  the  pulley  is 
called  a  fixed  pul- 
ley ;  and  when  the 
block  is  movable, 
as  in  the  case  of 
the  lower  pulley, 
A,  in  the  same 
figure,  the  pulley 
is  called  a  mova- 
ble pulley. 

Fig.  29. 

69.  Inclined  Plane.  —  An  inclined  plane  is  sim- 
ply an  inclined  surface  (Fig- 
ure 34). 

Illustrations.  —  The  skids,  or  the 
plank     used     for    loading    logs    or 
barrels   on   wagons,   and   the   plank 
by   which    a    steamer  is   loaded   or 
Flg-  3°-  unloaded,  and  by  which  barrels  are 

rolled  upon  a  platform  (Figure   35),  are  all  inclined  planes. 


Fig.  31. 


The  wedge  (Figure  36)  is  an  inclined  plane  which  is  driven 


52  NATURAL  PHILOSOPHY. 

under  the  weight  to  be   raised,  or  between  the  parts  to  be 


Fig.  32. 

separated.  The  screw  is  an  inclined  plane  in  which  the 
inclined  surface  winds  around  a  cylin- 
der in  the  form  of  a  thread.  The 
screw  turns  in  a  block,  N  (Figure  37), 
called  the  nut.  Sometimes  the  screw 
is  fixed  and  the  nut  movable,  and  some- 
times  the  nut  is  fixed  and  the  screw 
movable. 


7O.  Reasons  for  Using  Ma- 
chines.—  One  of  the  objects  to 
be  attained  by  a  machine  is  to 
transmit  the  power  from  one  point 
to  another,  or  to  change  the  point 
"Fig.  33.  at  which  the  power  acts.  This 

is  done  by  means  of  cords,  belts,  and  rods. 

A  second  end  to  be 
accomplished  by  the  use 
of  a  machine  is  that  of 
changing  the  direction  in 
^vhich  the  power  acts.  Fig.  34. 

Illustration.  —  By  means  of   two    fixed   pulleys,  as   shown 


NATURAL  PHILOSOPHY. 


53 


Fig.  35- 


in  Figure  38,  the  power  of  a  horse  is  made  to  act  upon  the 

weight    by   means 

of    the    rope,   and, 

the  horizontal  pull 

of     the     horse     is 

changed    into     an 

upward  pull  upon 

the  weight. 

A  third    end 

to    be    attained 

by    a    machine 

is  to  make  the  power  act  through  a  different  distance 
from  that  through  which  the  weight  is 
raised,  or  to  change  the  velocity  with 
which  the  power  acts  upon  the  weight, 
or  the  resistance  to  be  overcome.  The 
velocity  with  which  the  power  acts  may 
be  changed  by  the  use  of  pulleys  and 
wheels. 

Illustrations.  —  By  the  use  of  two  pulleys, 
one  in  the  movable  and  one  in  the  fixed  block 
(Figure  39),  the  power  which  is  at  the 
end  of  the  cord  will  have  to  move 
two  feet  in  order  to  raise  the  weight 
one.  By  the  use  of  six  pulleys,  three 
in  the  movable  and  three  in  the  fixed 
block,  as  shown  in  Figure  40,  the 
power  can  be  made  to  move  six  feet 
in  order  to  raise  the  weight  one. 

By  the  use  of  a  windlass  and  a 
fixed  pulley  in  a1  frame,  as  shown  in 
Figure  41,  the  power  applied  to  the 
spokes  or  to  a  crank  may  be  made 
to  raise  a  weight  vertically.  In  this 
case,  both  the  direction  and  the  velocity  of  the  power  arc 


Fig.  37- 


54 


NATURAL  PHILOSOPHY. 


changed.  The  power  will  have  to  move  over  the  circumference 
described  by  the  end  of  the  crank  in  order  to  raise  the 
weight  the  circumference  of  the  drum. 

In  the  crab  (Figure  42),  the 
•power  applied  to  the  cranks 
CC  turns  a  small  cog-wheel  P, 
called  the  pinion,  which  acts  upon 
the  toothed  wheel  W,  so  as  to 
turn  the  barrel  D,  to  which  the 
weight  is  attached.  In  this  case 
the  power  must  act  through  the 
circumference  described  by  the 
end  of  the  crank  in  order  to  turn 
the  pinion  around  once;  and  the 
pinion  must  turn  several  times  in 
order  to  turn  the.  barrel  once. 
The  crab  may  be  used  in  the  frame 
of  Figure  41,  instead  of  a  single 
windlass. 

In  the  derrick  (Figure  43)  the 
Fig.  38-  crab  is  used  at  the  bottom  of  the 

mast,  in  connection  with  a  system  of  pulleys  suspended  from 
the  end  of  the  boom.  In  this  case,  the  rate  at  which  the  power 
acts  upon  the  weight  is  diminished  still  more  than  in  the  last. 

In  the  crane,  shown  in  Figure  44,  the  crab 

is  replaced  by  a  more  powerful  piece  of  mechan- 
ism, shown  in  Figure  45.  The  barrel  A,  to 
which  the  rope  or  chain  is  attached,  is  turned 
by  the  cog-wheel  B :  this  wheel  is  turned  by  the 
pinion  C.  This  pinion  is  on  the  same  axle  as 
the  pinion  E,  which  is  turned  by  the  cog-wheels 
D  and  F.  These  cog-wheels  are  turned  by  the 
pinions  L  and  K j  which,  finally,  are  turned  by 
the  power  applied  to  the  cranks  L  and  K. 

Whenever,  in  wheel-work,  a  small  wheel  is  made 
to  act  upon  a  large  one,  the  rate  at  which  the 
power  acts  upon  the  resistance  is  diminished  ;  and, 


NATURAL  PHILOSOPHY. 


55 


Fig.  40. 


Fig.  4*. 


NATURAL  PHILOSOPHY. 


whenever  a  large  wheel  acts  upon  a  small  one, 
the  rate  at  which  the  power  acts  upon  the  weight 
is  increased. 

71.  The  Amount  of  Work  done  by  a  Machine. 
—  The  ^vork  done  by  a  machine  is  always  exactly 
equal    to    the   energy  expended  by    the  poiver   upon 
the  machine.     A  machine  is  not  an  instrument  for 

creating  power,  but  sim- 
ply for  transforming  the 
energy  of  the  power 
into  work.  The  work 
done  by  a  machine  is 
partly  the  useful  work 
of  raising  a  weight  or 
of  overcoming  a  resist- 
ance, and  partly  the 
useless  work  of  over- 
coming the  friction  and 
other  resistance  to  mo- 
tion in  the  machine 
itself.  The  useful  work 
clone  by  a  machine  is 
never  quite  equal  to  the 
J  energy  expended  upon 
the  machine.  The  use- 
less work  is  lessened  by  keeping  the  parts  of  the 
machine  well  oiled,  so  as  to  diminish  friction  as 
much  as  possible. 

72.  The  Great  Law  of  Machines.  —  The  energy 
expended  upon  a  machine  is    equal    to   the    power 
multiplied  by  the  distance  through  which  it  moves. 
The  work  done  by  a  machine  is  equal  to  the  weight 


Fig.  43- 


NATURAL  PHILOSOPHY. 


57 


multiplied  by  the  distance  which  it  is  raised. 
Hence  the  power  multiplied  by  the  distance  through 
which  it  acts  is  equal  to  the  weight  multiplied  by 
the  distance  it  is  raised.  This  is  the  great  funda- 
mental law  of  machines  ;  but  it  is  strictly  applica- 
ble only  to  an  ideal  machine  ; 
that  is,  to  one  which  would 
itself  offer  no  resistance  to 
motion. 


Fig.  44. 


73-  Gain  and  Loss  of  Power.  —  When  the  power 
moves  faster  than  the  weight,  there  is  said  to  be 
a  gain  of  power;  since  the  power  then  moves  a 
weight  greater  than  itself.  The  faster  the  power 
moves,  compared  with  the  weight,  the  greater  the 
gain  of  power,  or  the  larger  the  weight  that  will 
be  raised  by  the  power.  When  the  power  moves 


$8  NATURAL  PHILOSOPHY. 

slower  than  the  weight,  there  is  said  to  be  a  loss 
of  power ;  since  the  power  then  moves  a  weight 
smaller  than  itself. 

Whenever  there  is  a  gain  in  power,  there  is  an 
equal  loss  in  speed ;  and,  whenever  there  is  a  loss 
of  power,  there  is  an  equal  gain  in  speed. 

QUESTIONS. 

i.  What  is  a  machine  ?  2.  What  do  we  mean  by  the 
power  ?  3.  What  is  the  work  done  by  a  machine  ?  4.  Of 
what  elements  are  all  machines  composed  ?  5.  What  are  these 
elements  called  ?  6.  What  is  a  lever?  7.  Name  and  describe 
the  different  orders  of  levers.  8.  Give  illustrations  of  levers. 
9.  What  is  the  wheel  and  axle  ?  10.  How  are  the  weight  and 
power  applied?  n.  Give  illustrations.  12.  How  are  wheels 
and  axles  combined  in  mill-work?  13.  What  are  the  two 
modifications  of  the  wheel  and  axle  ?  14.  What  is  an  inclined 
plane?  15.  Give  illustrations.  16.  What  are  the  two  modifi- 
cations of  the  inclined  plane?  17.  What  is  one  of  the  ends 
to  be  attained  by  the  use  of  a  machine  ?  18.  By  what  means  is 
this  end  attained  ?  19.  What  is  a  second  end  to  be  attained  ? 
20.  Give  an  illustration.  21.  What  is  a  third  end  ?  22.  By 
what  means  is  this  end  attained  ?  23.  Give  the  illustration  of 
the  pulleys.  24.  Of  the  windlass  and  pulleys.  25.  Of  the 
crab.  26.  Of  the  derrick.  27.  Of  the  crane.  28.  When,  in 
wheel-work,  is  the  rate  at  which  the  power  acts  upon  the  weight 
increased,  and  when  diminished?  29.  To  what  is  the  work 
done  by  a  machine  always  equal  ?  30.  What  are  the  two 
kinds  of  work  done  by  a  machine?  31.  What  is  true  of  the 
amount  of  the  useful  work  ?  32.  How  may  the  useless  work 
be  lessened  ?  33.  What  is  the  great  law  of  machines  ?  34.  To 
what  kind  of  a  machine  alone  is  this  law  strictly  applicable? 

35.  When  is  there  said  to  be  a  gain  of  power  in  a  machine? 

36.  Why?     37.  When  is  there  said  to  be  a  loss   of  power? 
38.  Why  ?     39.  What  is  needed  to  make  a  small  power  raise  a 
very  large  weight  ?    40.  What  is  true  of  the  gain  or  loss  of 
speed  in  any  case  ? 


III. 

STATES    OF    MATTER. 


CHAPTER    XL 
FLUIDS. 

74.  The  Three  States.  —  Matter  exists   in   three 
different  states,  known  as  the  solid,  the  liquid,  and 
the  gaseous. 

Illustrations.  —  Ice  is  a  solid,  water  is  a  liquid,  and  steam 
and  air  are  gases. 

75.  Difference     between    the    Three    States.  - 

The  chief  difference  between  the  three  states  of 
matter  is  in  the  strength  of  cohesion,  or  attraction, 
among  the  molecules.  This  is  comparatively  strong 
in  solids,  very  weak  in  liquids,  and  entirely  want- 
ing in  gases. 

In  solids  the  molecules  have  certain  fixed  limits 
within  which  they  move  with  freedom,  but  from 
which  they  are  unable  to  escape.  In  liquids  the 
molecules  move  about  throughout  the  entire  mass 
with  the  utmost  freedom  ;  but  they  never  escape 
from  the  influence  of  cohesion.  In  gases  the  mole- 
cules are  not  under  the  slightest  restraint  from 

59 


60  NATURAL   PHILOSOPHY. 

cohesion  :  hence  they  move  in  straight  lines.  They 
are  continually  striking  together,  and  rebounding 
again ;  but  after  each  rebound  they  move  on  in 
straight  lines  till  they  encounter  other  molecules. 

If  a  piece  of  a  solid  is  placed  in  an  empty 
vessel,  it  will  either  retain  its  own  shape  com- 
pletely, as  in  the  case  of  a  stone,  or  else  conform 
to  the  shape  of  the  vessel  slowly  and  imperfectly, 
as  in  the  case  of  pitch  or  wax.  If  a  small  amount 
of  a  liquid  is  put  into  an  empty  vessel,  it  will 
conform  to  the  shape  of  the  vessel  at  once  and 
perfectly ;  but  it  will  not  fill  the  vessel.  It  will 

sink  into  the  lowest 
part,  and  be  separated 
by  a  definite  surface 
from  the  space  above. 
If  any  portion  of  a  gas, 
however  small,  is  placed 

Fig.  46.  .  , 

in      an      empty     vessel, 
however  large,  it  will  completely  fill  the  vessel. 

76.  Fluids.  —  On  account  of  the  freedom  of  their 
molecular   motion,    and    the    readiness   with   which 
their  parts  flow  over  or  among  each   other,  gases 
and  liquids  are  frequently  classed  together  as  fluids, 
They  have  several  characteristics  in  common. 

77.  Pascal's  Law.  —  One  of   the  most   remarka- 
ble   characteristics  of   a  fluid  is  the  way  in  which 
it  transmits  pressure.     If  any  pressure  is  brought  to 
bear  on  any  portion  of  tJie  surface  of  a  fluid  which 
fills  a  closed  vessel,  a  pressure  just  equal  to  it  will 
be  transmitted  through  the  fluid  to  every  equal  por- 
tion of  surface.     This  law  was  first  stated  by  Pascal, 
and  is  therefore  known  by  his  name. 


NATURAL  PHILOSOPHY. 


61 


Illustrations.  —  A  tube  (Figure  46)  provided  with  a  piston 
is  fitted  into  a  hollow  globe  which  is  pierced  with  a  number 
of  holes  in  a  circle  around  it.  Fill  the  globe  and  tube  with 
water,  and  push  in  the  piston.  The  water  spouts  out  of  all 
the  holes  with  equal  force. 


Fig.  47- 

The  hydraulic  press,  one  form  of  which  is  shown  in  Figures 
47  and  48,  is  an  illustration  of  Pascal's  law.  It  consists  of 
two  cylinders,  A  and  B,  one  large  and  the  other  small,  con- 
nected by  the  pipe  d.  The  piston  a  in  the  cylinder  A  is 
worked  by  the  handle  O,  and  forces  water  into  the  large 
cylinder  B,  where  it  presses  up  the  piston  C.  If  the  end  of 
the  piston  C  is  a  thousand  times  as  large  as  that  of  the 


62 


NATURAL  PHILOSOPHY, 


piston  a,  a  pressure  of  two  pounds  on  a  would  exert  a  press- 
ure of  two  thousand  pounds,  or  one  ton,  upon  C.  If  a  man,  in 
working  the  handle  (9,  forces  down  the  piston  a  with  a  pressure 
of  fifty  pounds,  he  would  bring  to  bear  upon  C  a  pressure  of 
twenty-five  tons. 

This  press,  which  is  one  of  the  most  powerful  machines 
ever  constructed,  is  used  for  pressing  cotton,  hay,  cloth,  etc., 
into  bales ;  for  extracting  oil  from  seeds ;  for  testing  cannon, 
boilers,  etc. ;  and  for  raising  ships  out  of  the  water. 


78.  The  Principle  of  Archimedes.  —  A  body  in 
a  fluid  is  buoyed  up  by  a  force  equal  to  the  weight 
of  the  fluid  it  displaces.  This  fact  was  discovered 
by  the  ancient  philosopher  Archimedes. 

Illustration.  —  Take  a  cup  and  a  cylinder  which  just  fills 
it,  hang  them  to  one  pan  of  a  balance  (Figure  49),  and  coun- 
terpoise them  in  the  air  by  adding  weights  to  the  other  pan. 
Hang  the  cylinder  in  a  vessel  of  water,  as  shown  in  the  figure. 
The  water  lifts  the  cylinder  up.  Fill  the  cup  with  water,  and 
the  beam  will  again  become  horizontal.  The  cup  holds  just 
as  much  water  as  the  cylinder  displaces. 


NATURAL   PHILOSOPHY.  63 

79.  To  Find  the  Weight  of  a  Body's  Bulk  of 
a  Liquid.  —  To  find  the  weight  of  a  body's  bulk 
of  a  liquid,  first  hang  the  body  to  one  pan  of  a 
balance,  and  counterpoise  it  by  weights  in  the  other 
pan.  Then  hang  it  in  water,  and  add  weights  to 
its  side  till  the  beam  is  again  horizontal.  The 


Fig.  49. 

weights  added  will  be  equal  to  the  weight  of  the 
body's  bulk  of  water. 

Illustrations.  —  Hang  a  piece  of  copper  to  one  pan  of  a 
balance,  and  counterpoise  it  by  weights  in  the  other  pan. 
Then  hang  it  in  water,  as  shown  in  Figure  50,  and  add  weights 
to  its  pan  till  it  sinks  into  the  water  enough  to  make  the 
beam  again  horizontal.  Suppose  it  takes  fourteen  grains  to 


64 


NATURAL   PHILOSOPHY. 


do  this :  the  weight  of  water  equal  in  bulk  to  the  copper  is 
then  fourteen  grains.  Suppose  the  piece  of  copper  weighs 
a  hundred  and  twenty-three  grains :  the  copper  is  then  about 
eight  times  and  eight-tenths  as  heavy  as  water. 

Counterpoise  a  glass  globe  containing  mercury  in  air,  in 
the  same  way  as  before,  and  then  hang  the  globe  in  water, 
as  shown  in  Figure  51,  and  add  weights  to  the  pan  to  which 


Fig.  50.  Fig.  51. 

the  globe  is  hung,  till  equilibrium  is  restored.  Suppose  it 
takes  forty-four  grains  to  do  this :  the  weight  of  the  globe's 
bulk  of  water  is  then  forty-four  grains.  Again :  counterpoise 
the  ball  in  air,  and  then  hang  it  in  alcohol,  and  add  weight 
to  its  pan  till  equilibrium  is  restored.  It  will  take  thirty-five 
grains  to  do  this.  The  weight  of  the  globe's  bulk  of  alcohol 
is  therefore  thirty-five  grains.  Alcohol  is  thus  seen  to  be 
about  eight-tenths  as  heavy  as  water. 


NATURAL  PHILOSOPHY. 


The  iv eight  of  a  substance,  compared  with  the 
weight  of  its  bulk  of  pure  water  at  a  temperature 
of  sixty  degrees,  is  called  its  specific  gravity. 

80.  Forces  Acting  upon  a  Body  immersed  in 
a  Fluid.  —  A  body  when  immersed  in  a  fluid  is 
acted  upon  by  two  forces,  —  one  equal  to  its  own 
weight,  ^which  tends  to  make  the  body  sink ;  and 
one  equal  to  the  weight  of  the  fluid  displaced, 
which  tends  to  make  the  body  rise.  When  a  body 
displaces  more  than  its  own  weight  of  a  fluid,  it 


will  rise  in  that  fluid  ;  when  it  displaces  less  than 
its  own  weight,  it  will  sink ;  and,  when  it  displaces 
just  its  own  weight^  it  will  remain  suspended  wher- 
ever it  happens  to  be. 

Illustrations.  —  An  egg  placed  in  salt  water  (Figure  52) 
rises  to  the  surface,  because  it  displaces  more  than  its  own 
weight  of  the  brine.  When  it  is  put  into  fresh  water,  it 
sinks  to  the  bottom,  because  it  displaces  less  than  its  own 
weight  of  the  water.  When  it  is  put  into  a  proper  mixture  of 
fresh  water  and  brine,  it  will  remain  suspended  in  the  fluid, 
because  it  displaces  just  its  own  weight  of  the  mixture. 


66 


NATURAL  PHILOSOPHY, 


8 1.  Floating  Bodies.  —  Every  body  floating  in  a 
fluid  displaces  its  own  weight  of  the  fluid. 

Illustrations.  —  A  ship  displaces  its  own  weight  of  water. 
The  heavier  the  ship  is  loaded,  the  deeper  she  sinks  into  the 
water.  A  solid  piece  of  iron  sinks  in  water;  but  when  the  iron 


Fig.  54- 

is  shaped  into  a  hollow  vessel, 
as  in  the  case  of  iron  ships,  the 
vessel  will  float  on  the  water, 
since  it  will  then  displace  its 
own  weight  of  water  before  it 
sinks  to  the  surface. 

A  balloon  (Figure  53)  displaces 
its  own  weight  of  air.  By  throwing  out  the  sand  which  is 
used  as  ballast,  the  balloon  is  made  lighter,  so  as  to  displace 
more  than  its  own  weight  of  air.  It  then  rises  till  it  comes 
into  more  highly  rarefied  air,  where  it  displaces  just  its  own 
weight,  when  it  again  floats  along  at  the  same  level.  If  some 


Fig.  53- 


NATURAL  PHILOSOPHY.  67 

of  the  gas  is  allowed  to  escape,  the  balloon  becomes  less  in 
bulk,  and  so  displaces  less  than  its  own  weight  of  air.  It  then 
sinks  until  it  again  displaces  its  own  weight. 

Hydrometers  usually  consist  of  a  glass  tube  with  a  bulb 
blown  upon  it,  and  weighted  at  the  bottom.  Common  forms 
of  this  instrument  are  shown  in  Figure  54.  When  put  in 
a  liquid,  they  sink  in  it  till  they  displace  their  own  weight. 
The  deeper  they  sink  in  a  liquid,  the  less  its  specific  gravity. 
Their  stems  are  graduated  in  such  a  way,  that  the  number  on 
the  stem  at  the  surface  of  the  liquid  indicates  the  specific 
gravity  of  the  liquid. 

QUESTIONS. 

i.  Name  the  three  states  of  matter.  2.  Give  an  illustration 
of  each.  3.  What  is  true  of  the  strength  of  cohesion  in  each  ? 
4.  What  is  true  of  the  molecular  motion  of  each  ?  5.  What  is 
the  result  when  a  small  portion  of  each  is  placed  in  an  empty 
vessel  ?  6.  What  two  states  of  matter  are  often  classed  together 
as  fluids  ?  7.  Why  ?  8.  What  is  one  of  the  most  remarkable 
characteristics  of  a  fluid?  9.  State  Pascal's  law.  10.  Give  the 
illustration  of  the  hollow  globe,  n.  Describe  the  hydraulic 
press.  12.  Describe  its  action.  13.  What  are  some  of  its 
uses?  14.  State  Archimedes's  principle.  15.  How  came  it  to 
be  designated  by  his  name?  16.  Give  the  illustration  of  the 
cylinder  and  cup.  1 7.  How  can  we  find  the  weight  of  a  body's 
bulk  of  a  fluid?  18.  Give  the  illustration  of  the  piece  of 
copper.  19.  Of  the  glass  globe.  20.  What  do  we  mean  by 
the  specific  gravity  of  a  substance?  21.  To  what  two  forces 
is  a  body  immersed  in  a  fluid  subjected  ?  22.  When  will  such 
a  body  sink  ?  23.  When  will  it  rise  ?  24.  When  will  it  remain 
suspended  in  the  fluid?  25.  Give  the  illustration  of  the 
egg.  26.  How  much  of  a  fluid  is  displaced  by  a  floating 
body?  27.  Give  the  illustration  of  a  ship.  28.  Of  an  iron 
ship.  29.  Of  a  balloon.  30.  Of  a  hydrometer. 


68 


NATURAL  PHILOSOPHY. 


CHAPTER    XII. 

PROPERTIES    OF    GASES,    LIQUIDS,    AND    SOLIDS. 

82.  Expansibility  of  Gases. — The  most  remarka- 
ble property  of  a  gas  is  its  capacity  for  indefinite 
expansion.  This  is  due  to  the  absence  of  cohesion 
and  to  the  rapidity  with  which  its  molecules  are 
moving  in  straight  lines. 

Illustration.  —  Put  a  rubber  bag,  partially  filled  with  air, 
under  the  receiver  of  an  air-pump,  and  exhaust  the  air  from 

the  receiver.  As  the  air 
becomes  exhausted,  the  bag 
fills  out,  as  shown  in  Fig- 
ure 55. 

Explanation.  —  It  is  esti- 
mated that  the  molecules  of 
air  are  moving  at  the  rate 
of  over  sixteen  hundred  feet 
a  second.  In  a  cubic  yard 
of  air  there  are  two  pounds 
of  these  molecules.  The 
expansive  force  of  a  cubic 

yard  of  air  is  therefore  equal  to  the  force  of  a  two-pound 
cannon-ball  moving  at  the  rate  of  sixteen  hundred  feet  a 
second,  or  of  a  mile  in  three  seconds  and  a  half.  Could  all 
the  motions  of  the  molecules  of  the  air  be  turned  into  one 
and  the  same  direction,  the  result  would  be  a  hurricane  sweep- 
ing over  the  earth,  whose  destructive  violence  not  even  the 
Pyramids  could  withstand.  "  Living,  as  we  do,  in  the  midst 
of  a  molecular  tornado  capable  of  such  effects,  our  safety  lies 
wholly  in  the  circumstance  that  the  storm  beats  equally  in  all 
directions  at  the  same  time :  and  the  force  is  thus  so  exactly 
balanced,  that  we  are  wholly  unconscious  of  the  tumult." 


83.   Diffusion  of  Gases.  —  When    any  two  gases 


NATURAL  PHILOSOPHY.  69 

are  brought  into  contact,  they  rapidly  mix  with 
each  other.  This  mixture  of  gases,  when  brought 
into  contact,  is  called  diffusion.  It  is  due  to  the 
fact  that  the  molecules  are  far  apart  and  in  con- 
stant motion.  The  molecules  of  the  one  gas  quickly 
move  into  the  spaces  among  the  molecules  of  the 
other  gas. 


Fig.  56. 

84.  The  Air- Pump.  —  A  common  form  of  air- 
pump  is  shown  in  Figure  56,  and  the  internal 
structure  of  a  similar  one  in  Figure  57.  It  con- 
sists of  a  flat  plate  for  holding  the  receiver  E. 
The  plate  is  connected  by  a  tube  with  a  cylinder, 
in  which  a  piston  is  moved  up  and  down  by  means 
of  the  handle.  There  is  a  valve  5  in  the  piston, 
which  opens  upwards,  and  which  is  pressed  down 


70  NATURAL  PHILOSOPHY. 

by  a  little  spiral  spring  above  it.  There  is  a  sec- 
ond valve  S'  at  the  bottom  of  the  cylinder,  which 
is  fastened  to  the  end  of  a  rod  passing  through 
the  piston.  When  the  piston  is  drawn  up,  this 
valve  is  opened  by  the  friction  of  the  rod  in  the 
piston  ;  and,  when  the  piston  is  pushed  down,  this 
valve  is  closed  by  the  same  means.  The  pump- 
plate  and  the  mouth  of  the  receiver  are  both  ground 
flat,  so  as  to  form  an  air-tight  joint.  As  the  piston 


Fig.  57- 

is  drawn  up,  the  air  in  the  receiver  expands,  and 
a  portion  of  it  passes  out  through  the  open  valve 
S'  into  the  cylinder  behind  the  piston.  When  the 
piston  is  again  pushed  down,  the  valve  S'  is  closed ; 
the  valve  5  is  opened  by  the  pressure  of  the  air 
below  it,  and  this  air  passes  through  it  into  the 
space  above  the  piston.  As  the  piston  is  again 
raised,  more  air  passes  from  the  receiver  into  the 
cylinder ;  and  so  on,  till  the  air  is  nearly  all  ex- 
hausted from  the  receiver. 

85.   Liquids  are  nearly  Incompressible.  —  Gases 
are  readily  compressed,  but  liquids  are  nearly  incom- 


NATURAL  PHILOSOPHY.  7\ 

pressible.  This  is  one  of  the  most  essential  points 
of  difference  between  a  liquid  and  a  gas.  It  was 
thought  for  a  long  time  that  liquids  were  entirely 
incompressible. 

Illustration.  —  In  the  year  1661  some  Florentine  philoso- 
phers, wishing  to  ascertain  whether  water  was  compressible, 
filled  a  thin  globe  of  gold  with  this  liquid,  closed  it  perfectly 
tight,  and  then  subjected  it  to  an  enormous  pressure  in  order 
to  flatten  the  globe  so  as  to  diminish  its  capacity.  They  failed 
to  compress  the  water,  but  discovered  the  porosity  of  gold  (19); 
for  the  water  forced  its  way  through  the  pores  of  the  gold, 
and  stood  on  the  outside  like  dew. 

86.  Liquids  tend  to  Assume  a  Globular  Form. 
—  When  left  to  itself,  a  liquid  always  assumes   a 

globular  form. 

• 

Illustrations.  —  Prepare  a  mixture  of  water  and  alcohol 
which  shall  be  just  as  heavy  as  sweet  oil,  bulk  for  bulk,  and 
introduce  some  of  the  oil  carefully  into  the  centre  of  this 
mixture  by  means  of  a  dropping-tube :  the  oil  will  neither  rise 
nor  sink,  but  gather  into  a  beautiful  sphere. 

Rain-drops,  dew-drops,  and  the  manufacture  of  shot,  illus- 
trate this  tendency  of  the  molecules  of  liquids.  In  the  manu- 
facture of  shot,  melted  lead  is  poured  through  a  sieve  at  the 
top  of  a  very  high  tower ;  and  the  drops,  in  falling,  take 
the  form  of  spheres,  which  become  solid  before  they  reach 

the  bottom. 

*  . 

87.  The   Pressure   of  a  Liquid   is  Proportional 
to  the  Depth.  —  The  pressure  of   a  liquid  at   any 
point    is   proportional  to    the    depth    of    the    liquid 
above   that    point,  and   entirely  independent  of  the 
quantity  of  the  liquid  above  it. 

Illustration.  —  In  Figure  58  is  shown  a  strong  cask  having 
a  small  tube  thirty  or  forty  feet  high  fastened  into  its  top. 


NATURAL  PHILOSOPHY. 


If  water  is  poured  into  the  tube  so  as  to  till  the  cask  and 
tube,  the  pressure  at  the  bottom  of  the  cask  will  be  sufficient 
to  burst  the  cask.  The  pressure  on  the  bottom  of  the  cask 
is  the  same  that  it  would  be,  were  the  cask  itself  thirty  or 
forty  feet  high,  and  filled  with  water. 

88.  Rise  of  Liquids  in  Commu- 
nicating Vessels.  —  When  a  liquid 
is  contained  in  a  series  of  vessels 
connected  with  each  other,  the  liquid 
will  rise  to  the  same  height  in  all 
the  vessels,  no  matter  what  may  be 
their  size  or  shape,  as  shown  in 
Figure  59. 

Illustrations.  —  Water  will  rise  in  all 
the  pipes  connected  with  a  reservoir  to 
the  height  of  the  level  of  the  water  in  the 
reservoir.  If  the  upper  stories  of  any  of 
the  houses  are  above  the  level  of  the  water 
in  the  reservoir,  the  water  will  not  rise  to 
those  stones. 

Artesian  wells  and  deep-seated 
springs  are  illustrations  of  this  same 
fact.  These  are  openings  into  inter- 
nal reservoirs  of  water  which  are 
somewhere  in  communication  with 
water  at  a  higher  level  than  the 
spring,  or  the  mouth  of  the  well. 
Springs  are  natural  openings  into 
such  reservoirs,  and  Artesian  wells 
Fig-  58.  are  artificial  openings.  An  Artesian 

well  is  shown  in  Figure  60.  AB  and  CD  are  two  layers 
which  water  cannot  penetrate.  The  space  between  these  is 
supposed  to  be  filled  with  water  above  the  level  of  the  sur- 
face at  H :  hence  the  water  rises  through  the  well,  and  over- 
flows at  the  mouth.  Artesian  wells  are  often  bored  several 
thousand  feet  deep. 


NATURAL  PHILOSOPHY. 


73 


89.  Solids  tend  to  assume  a  Crystalline  Struc- 
ture.—  Whenever  a  solid  is  formed  under  circum- 


Fig-  59- 


stances  which  leave  the  molecules  free  to  arrange 
themselves   as   they  will,    it   assumes   a   crystalline 


Fig.  60. 


structure,  or  one  made  up  of  crystals.  These  are 
regular  geometrical  forms,  which  are  different  for 
different  substances. 


74  NATURAL  PHILOSOPHY. 

Illustrations.  —  Snowflakes,  when  examined  with  the  micro- 
scope, are  seen  to  be  composed  of  beautiful  crystals,  as  shown 
in  Figure  61.  The  same  is  true  of  the  delicate  frost  on  the 
window-pane. 

90.  Properties  of  Solids.  —  A  body  is  said  to  be 
tenacious  when  it  is  difficult  to  pull  it  in  two.  All 


Fig.  61. 

solids  are  more  or  less  tenacious,  but  they  differ 
much  in  the  degree  of  their  tenacity.  A  body  is 
said  to  be  hard  when  it  is  difficult  to  scratch  or 
indent  it;  that  is  to  say,  when  it  is  difficult  to 
displace  its  molecules.  All  solids  are  elastic  within 
certain  limits;  and  this  elasticity  may  be  devel- 
oped by  stretching,  bending,  twisting,  compression, 
or  any  kind  of  strain  whatever.  Different  solids, 
however,  differ  greatly  in  the  degree,  or  limit,  of 


NATURAL   PHILOSOPHY. 


75 


their  elasticity.  When  the  strain  is  carried  beyond 
this  limit,  the  body  must  either  break,  or  take  a  new 
form.  A  body  'which  is  apt  to  break  ^cvken  strained 
beyond  the  limit  of  elasticity  is  said  to  be  brittle. 
A  brittle  substance  is  not  always  easily  broken. 
It  is  often  difficult  to  strain  it  beyond  the  limit 
of  its  elasticity.  It  is  not  easy  to  break  a  glass 
rod  an  inch  in  diameter ;  yet  glass  is  the  most 
brittle  substance  known.  Substances  which  can 
readily  take  and  retain  new  forms  are  said  to  be 
malleable  or  ductile.  A  malleable  substance  is  one 
that  can  be  hammered  or  rolled  into  sheets,  and  a 
ductile  substance  one  that  can  be  drawn  into  wire. 
All  malleable  sub- 
stances are  to  some 
extent  ductile ;  but  the 
most  malleable  are  not 
the  most  ductile. 

Illustration.  —  Gold    is 
one  of  the  most  malleable  Fig"  62'  Fig"  63> 

of  the  metals.  In  the  manufacture  of  gold-leaf  it  is  ham- 
mered out  into  sheets  so  thin  that  it  takes  from  three  hun- 
dred thousand  to  three  hundred  and  fifty  thousand  of  them 
to  make  the  thickness  of  a  single  inch. 

91.  Capillarity.  —  When  a  liquid  wets  a  vessel 
which  holds  it,  as  in  the  case  of  water  in  a  goblet, 
the  water  rises  a  little  on  the  sides  of  the  vessel, 
as  shown  in  Figure  62.  When  the  liquid  cannot 
wet  the  vessel,  as  in  the  case  of  quicksilver  in  a 
glass  goblet,  the  liquid  is  depressed  a  little  around 
the  sides  of  the  vessel,  as  shown  in  Figure  63. 

If  a  tube  is  plunged  into  a  liquid  which  will  wet 


76  NATURAL  PHILOSOPHY. 

it,  as  in  the  case  of  a  glass  tube  in  water,  the 
liquid  will  rise  higher  within  the  tube  than  it  is 
on  the  outside,  as  shown  in  Figure  64.  The  finer 
the  tube,  the  higher  the  liquid  rises  in  it.  If  a 

tube  is  plunged 
into  a  liquid  which 
will  not  wet  it,  as 
in  the  case  of  a 
glass  tube  in  quick- 
silver, the  liquid 
will  fall  lower  with- 
in the  tube  than 
is  it  is  on  the  out- 


Fi£-  6«-  side,  as    shown    in 

Figure  65.  The  finer  the  tube,  the  lower  the  liquid 
is  depressed. 

This  peculiar  action  of  liquids  in  contact  with 
the  surface  of  solids  is  called  capillarity,  because 
it  is  especially  manifest  in  capillary,  or  hair-like, 
tubes  (from  the  Latin 
capillus,  a  hair). 

Illustrations.  —  A  lamp- 
wick  is  full  of  tubes  and 
pores;  and  capillary  force 
draws  the  oil  up  through 
these  to  the  top  of  the 
wick,  where  it  is  burned. 
When  one  end  of  a  cloth 

is  put  into  water,  capillary  force  draws  the  water  into  the 
tubes  and  pores  of  the  cloth,  and  the  whole  soon  becomes 
wet.  In  the  same  way,  any  other  porous  substance  soon 
becomes  wet  throughout,  if  a  corner  of  it  is  put  into  water. 
Blotting-paper  is  full  of  pores,  into  which  the  capillary  force 


NATURAL  PHILOSOPHY.  77 

draws  the  ink.  The  use  of  a  towel  for  wiping  any  thing  which 
is  wet  depends  on  the  same  principle.  Small  steel  needles 
will  float  on  water  when  placed  carefully  on  the  surface  (Figure 
66).  Some  insects  walk  on  water  (Figure  67).  In  all  these 
cases  the  bodies  are  not  wet  by  the  liquid,  and  consequently 


Fig.  67.  Fig.  68. 

depressions  are  formed  around  them  by  capillary  action,  as 
shown  in  Figure  68.  The  liquid  displaced  by  one  of  these 
bodies  is  really  equal  to  that  which  would  fill  the  whole  depres- 
sion, or  the  space  below  the  dotted  line  CD;  and  this  liquid 
would,  in  every  case,  be  equal  to  the  weight  of  the  floating 
body. 

QUESTIONS. 

i.  What  is  the  most  remarkable  property  of  a  gas?  2.  To 
what  is  the  expansiveness  of  a  gas  due  ?  3.  Give  the  illustra- 
tion of  the  rubber  bag.  4.  At  what  rate  are  the  molecules  of 
air  moving?  5.  To  what  is  the  expansive  force  of  a  cubic 
yard  of  air  equal?  6.  What  would  be  the  result  were  the 
motions  of  all  the  molecules  turned  in  the  same  direction? 
7.  Why  do  we  not  feel  this  molecular  tornado  ?  8.  What  is 
meant  by  the  diffusion  of  gases?  9.  What  takes  place  in 
diffusion?  10.  Describe  the  air-pump,  n.  Explain  its  action. 
12.  What  is  one  of  the  most  essential  points  of  difference 
between  a  liquid  and  a  gas.  13.  Give  the  Florentine  experi- 
ment. 14.  What  form  do  liquids  tend  to  assume?  15.  Give 
illustrations  of  this  tendency.  16.  To  what  is  the  pressure 
of  a  liquid  proportional?  17.  Give  the  illustration  of  the  cask. 
18.  What  is  true  of  the  rise  of  liquids  in  communicating  ves- 
sels? 19.  Give  the  illustration  of  aqueducts.  20.  Of  springs 
and  Artesian  wells.  21.  What  structure  do  solids  tend  to 
assume  ?  22.  Give  illustrations.  23.  When  is  a  body  said  to 


78  NATURAL  PHILOSOPHY. 

be  tenacious  ?  24.  What  is  true  of  solids  as  regards  tenacity  ? 
25.  When  is  a  body  said  to  be  hard?  26.  What  is  true  of 
solids  as  regards  elasticity  ?  27.  When  is  a  body  said  to  be 
brittle?  28.  Is  a  brittle  substance  always  easily  broken? 

29.  When   is   a  substance  said  to  be  malleable    or    ductile? 

30.  What  is  the  difference  between  the  two?     31.  What  is  true 
of  the  malleability  of  gold  ?     32.  When  a  liquid  can  wet  the 
vessel  which  holds  it,  what  takes  place  along  the  side  of  the 
vessel?     33.  When  it  cannot  wet  it?     34.  What  takes  place 
when  a  tube  is  put  into  a  liquid  which  can  wet  it?     35.  Which 
cannot  wet  it  ?     36.  Does  the  size  of  the  tube  make  any  differ- 
ence ?     37.  What   name    is   given    to    this    action    of    liquids? 
38.  Give  illustrations  of  capillarity.     39.  Explain  how  needles 
and  other  heavy  bodies  can  be  made  to  float  on  water. 


CHAPTER    XIII. 

ATMOSPHERIC    PRESSURE. 

92.  Pressure  of  the  Air.  —  The  pressure  of  the 
air  may  be  illustrated  by  the  following  experiment : 
place  a  small  bell-jar,  open  at  both  ends,  on  the 
plate  of  the  air-pump,  and  cover  the  top  of  the 
jar  with  the  palm  of  the  hand.  When  the  air  is 
exhausted  from  the  jar,  the  hand  is  pressed  firmly 
down  upon  the  mouth  of  the  jar.  This  illustrates 
the  downward  pressure  of  the  air.  It  was  not  per- 
ceived at  first,  because  the  downward  pressure  of 
the  air  upon  the  hand  was  balanced  by  the  upward 
pressure  of  the  air  within  the  jar. 

The  pressure  of  the  air  at  the  level  of  the  sea 
is  about  fifteen  pounds  to  the  square  inch.  As 
we  ascend  in  the  atmosphere,  this  pressure  is  less 
and  less,  because  there  is  less  depth  of  air  above  us. 


NATURAL  PHILOSOPHY. 


79 


93.   Rise  of  Liquids  in  Exhausted  Tubes.  —  A 

liquid  will  rise  in  any  exhausted  tube  which  opens 
into  it.  The  height  to  which  the  liquid  will  rise 
varies  with  its  density.  Water  will  rise  somewhat 
over  thirty  feet,  and  mer- 
cury about  thirty  inches. 
The  rise  of  the  liquid 
in  such  a  tube  is  due 
to  the  pressure  of  the 
atmosphere  on  the  sur- 
face of  the  liquid  :  this  is 
about  fifteen  pounds  to 
the  square  inch.  When 
the  air  is  removed  from 
a  vessel  or  tube  which 
opens  into  a  liquid,  there 
would  be  no  pressure 
within  the  tube  to  bal- 
ance the  pressure  of  the 
atmosphere  on  the  out- 
side, if  the  liquid  did  not 
rise  in  the  tube  :  hence 
the  liquid  rises  in  the 
tube  till  its  pressure  is 
equal  to  fifteen  pounds  Fis-  69- 

to  the  square  inch,  or  to  that  of  the  atmosphere  on 
the  outside. 

Illustrations.  — A  glass  tube  closed  at  one  end,  and  some- 
what over  thirty  inches  in  length,  is  filled  with  mercury,  closed 
with  the  thumb,  and  inverted  in  a  cup  of  mercury,  as  shown 
in  Figure  69.  The  mercury  will  fall  in  the  tube  to  within 
about  thirty  inches  of  the  surface  of  the  mercury  in  the  cup. 


8o 


NATURAL  PHILOSOPHY. 


This  experiment  was  first  tried  by  an  Italian  named  Tom- 
celli :  hence  such  a  tube  is  known  as  a  Torricellian  tube,  and 
the  empty  space  above  the  column  of  mercury  as  a  Torricel- 
lian vacuum. 

In  drinking  lemonade  through  a  straw,  the  air  is  first  drawn 
out  of  the  straw  by  the  mouth,  and  the  liquid  is  forced  up 
through   the   straw  by  the  pressure  of 
the  air  on  the  surface. 

In  drawing  water  with  ah  ordinary 
pump,  the  air  is  first  removed  from  the 
barrel  and  tube  by  the  action  of  the 
piston;  and  the  water  is  then  forced 
up  through  the  tube  into  the  barrel  by 
the  pressure  of  the  atmosphere  on  the 
surface  of  the  water  in  the  well  or 
cistern.  The  two  kinds  of  pumps  in 
ordinary  use  are  shown  in  Figures  70 
and  71.  Each  has  a  valve  at  the  top 
of  the  tube,  which  communicates  with 
the  well  or  cistern.  The  first  has  a 
valve  in  the  piston,  which  moves  up  and 
down  in  the  barrel.  In  the  second  the 
piston  is  solid;  but  there  is  a  second 
valve  in  the  discharge-pipe,  which,  in 
this  case,  opens  from  the  bottom  of  the 
barrel.  With  the  former,  the  water  is 
lifted  out  of  the  barrel  every  time  the 
piston  is  raised;  while  with  the  latter 
the  water  is  forced  out  of  the  barrel 
every  time  the  piston  is  pushed  down. 
The  former  is  called  the  lifting-pump, 
or  suction-pump;  and  the  latter,  the 
force-pump.  With  the  former,  water  can  be  raised  only  about 
thirty  feet  high ;  while  with  the  latter  it  can  be  raised  to  any 
desired  height. 

In  either  pump,  as  the  piston  is  raised,  the  water  is  forced 
up  through  the  tube  and  the  valve  S,  into  the  barrel ;  and,  as 
the  piston  is  pushed  down,  this  valve  closes,  and  keeps  the 


Fig.  70. 


NATURAL  PHILOSOPHY. 


81 


water  in  the  barrel  from  passing  back  into  the  well.  The  valve 
O  at  the  same  time  opens,  and  allows  the  water  in  the  barrel 
to  pass  above  the  .piston  in  the  lifting-pump,  and  into  the  dis- 
charge-pipe of  the  force-pump.  When  the  piston  rises  again, 
the  valve  O  closes. 

94.  The  Siphon.  —  The  sip/ion  is  used  for  trans- 
ferring liquids  from  one  vessel  to 
another.  It  consists  of  a  bent  tube, 
with  arms  of  unequal  length  (Fig- 
ure 72).  The  air  must  be  removed 
from  the  tube  in  the  first  place, 
either  by  applying  the  mouth  to 
the  end  B,  after  the  other  arm  of 
the  siphon  has  been  put  into  the 
vessel  of  water,  or  by  filling  the 
siphon  with  water  before  it  is 
placed  in  the  vessel.  The  water 
will  flow  through  the  siphon  from 
C  to  B  until  the  vessel  is  emptied, 
or  until  the  level  of  the  water 
falls  below  the  mouth  of  the  arm  in  the  vessel. 

The  flow  of  the  liquid  through  the  siphon  seems 
opposed  to  the  well-known  fact  that  water  will  not 
run  up  hill  ;  but  it  will  be  seen  that  the  water  is 
really  flowing  from  a  higher  level  C  to  a  lower  level 
B.  If  we  consider  the  water  in  the  siphon  at  M, 
we  see  that  the  force  which  acts  upon  it  from  left 
.  to  right  is  equal  to  the  pressure  of  the  atmosphere 
minus  the  pressure  of  the  water  in  the  tube  from 
Mto  C,  whose  depth  is  DC;  and  the  pressure  which 
acts  upon  it  from  right  to  left  is  equal  to  the  pressure 
of  the  atmosphere  minus  the  pressure  of  the  water 


Fig.  71. 


82 


NATURAL  PHILOSOPHY. 


in  the  tube  from  M  to  B,  whose  depth  is  A  B.     Since 

AB  is  greater  than  D  C,  the  pressure  at  M  towards 

the  right  will  be  greater  than  that  towards  the  left. 

Consequently  the  water 
at  M  moves  on  towards 
B ;  and,  as  it  moves 
away,  more  water  is 
driven  up  into  the  arm 
CM  to  take  its  place, 
by  the  pressure  of  the 
atmosphere  on  the  sur- 
face of  the  water  in  the 
vessel.  No  liquid  will 
flow  through  a  siphon 
unless  the  atmospheric 
pressure  is  sufficient  to 

raise  it  to  the  bend  of  the  tube. 

95.  Tantalus's  Cup.  —  This   is  a  glass  cup  ivith 

a  siphon  tube  passing  through  the  bottom,  as  shown 

in  Figure  73.     If  water  is  poured  into  the  cup,  it 

will  rise  both  inside  and  out- 
side the   siphon,  until   it  has 

reached  the  top  of  the  tube, 

when    it    will   begin    to    flow 

out.     If   the  water  runs  into 

the  cup  less  rapidly  than  the 

siphon    carries  it  out,  it  will 

sink   in    the    cup    until    the 

shorter  arm    no    longer  dips 

into  the  liquid,  and  the  flow  from  the  siphon  ceases. 

The  cup  will  then  fill  as  before,  and  so  on. 

In  many  places   there  are  springs  which  flow  at 


Fig.  73- 


NATURAL   PHILOSOPHY  83 

intervals,  like  the  siphon  in  this  experiment,  and 
whose  action  may  be  explained  in  the  same  way. 
A  cavity  under  ground  (Figure  74)  may  be  gradu- 
ally filled  with  water  by  springs,  and  then  emptied 
through  an  opening  which  forms  a  natural  siphon, 
In  some  cases  of  this  kind  the  flow  stops  and 


Fig.  74- 

begins  again  several  times  in  an  hour.     Such  springs 
are  called  intermittent  springs. 

96.  The  Barometer. — The  barometer -is  an  instru- 
ment for  measuring  the  pressure  of  the  atmosphere 
In  its  ordinary  form  it  is  a  Torricellian  tube  fur- 
nished with  a  convenient  case  (Figure  75).  The 
vessel  for  the  mercury  at  the  bottom  must  be  con- 
structed so  as  to  prevent  the  spilling  of  the  mercury 


84  NATURAL  PHILOSOPHY. 

in  transportation,  and  so  as  to  allow  the  atmosphere 
to  act  freely  upoTi  the  mercury. 

A  change  in  the  weather  is  generally 
attended  with  a  change  in  the  pressure 
of  the  atmosphere  :  hence  the  rise  and 
fall  of  the  barometer  often  enable  us  to 
foretell  the  weather.  As  a  general  rule, 
the  rising  of  the  mercury  indicates  the 
coming  of  fair  weather,  and  its  falling 
that  of  foul  weather. 

If  we  take  a  barometer  up  a  moun- 
tain, the  mercury  will  fall,  because  there 
is  less  weight  of  air  pressing  upon  it. 
The  higher  we  go,  the  lower  will  the 
mercury  fall  :  hence  the  barometer  may 
be  used  to  measure  the  height  of  moun- 
tains. 

Illustrations.  —  At  the  level  of  the  sea  the 
height  of  the  column  of  mercury  in  the  barome- 
ter is  about  thirty  inches.  As  we  go  up  from 
this  level,  the  mercury  falls  about  one  inch  for 
every  nine  hundred  feet  of  perpendicular  height. 
As  the  density  of  the  air  diminishes  with  the 
ascent,  the  fall  of  the  mercury  for  a  given  differ- 
ence of  elevation  will  be  less  and  less  as  we  go 
higher  and  higher.  The  fall  is  also  affected  by 
the  temperature.  Tables,  however,  have  been 
prepared,  which  enable  us  to  get  the  height  of 
a  place  above  the  level  of  the  sea  quite  accu- 
Flg*  75'  rately  by  observing  the  fall  of  the  barometer 
and  the  change  in  temperature. 


NATURAL  PHILOSOPHY.  85 

QUESTIONS. 

I.  Give  an  illustration  of  the  pressure  of  the  atmosphere. 
2.  To  what  is  the  pressure  of  the  atmosphere  equal  at  the 
level  of  the  sea  ?  3.  What  js  true  of  it  as  we  ascend  ?  4. 
Why?  5.  To  what  height  will  water  rise  in  an  exhausted 
tube  ?  6.  To  what  height  will  mercury  rise  ?  7.  What  causes 
a  liquid  to  rise  in  exhausted  tubes  and  vessels  ?  8.  Why  will 
some  liquids  rise  higher  than  others  ?  9.  Give  Torricelli's 
experiment.  10.  What  is  the  action  in  drinking  lemonade 
through  a  straw?  11.  In  pumping  water ?  12.  What  are  the 
names  of  the  two  kinds  of  pumps  in  ordinary  use?  13.  Why 
are  they  so  named?  14.  Where  are  the  valves  in  each?  15. 
What  takes  place  when  the  piston  of  each  is  raised  and 
depressed?  16.  What  is  a  siphon  ?  17.  For  what  is  it  used? 
18.  How  is  the  flow  started?  19.  How  long  will  it  continue? 
20.  Explain  why  the  liquid  flows  through  the  siphon.  21.  De- 
scribe Tantalus's  cup.  22.  Describe  its  action.  23.  What  are 
intermittent  springs ?  24.  What  is  a  barometer?  25.  Of  wh.at 
does  it  consist  ?  26.  What  are  its  two  chief  uses  ?  27.  What 
is  said  of  its  use  in  measuring  the  height  of  mountains  ? 


IV. 

SOUND. 

CHAPTER    XIV. 

ORIGIN    AND    NATURE   OF   SOUND. 

97.  Origin  of  Sound.  —  Every  body  which  is 
emitting  sound  is  in  a  state  of  vibration.  When 
the  vibration  stops,  the  sound  ceases. 
These  vibrations  are  executed  either  by 
the  body  as  a  whole,  or  by  sensible 
portions  of  the  body  :  they  are,  therefore, 
molar  vibrations  (13).  Sound  originates 
in  molar  vibrations  of  solids,  liquids,  or 
gases. 


Illustrations.  —  Fill  a  glass  brimful  of  water, 
and  strike  a  tuning-fork  so  as  to  cause  it  to 
emit  a  sound.  Hold  the  edge  of  the  prong  of 
the  fork  in  contact  with  the  water :  a  shower 
of  spray  will  fly  off  on  each  side,  showing  that 
the  prongs  are  in  vibration. 

Pluck    one    of    the   strings  of   a  violin  so   as 
to  make  it  give  a  sound.     On  looking  directly 
Fig-  76          down  upon  the   string,  we  see    that   it   is   in   a 
state   of    vibration.     It   will   have    the   appearance   shown   in 
Figure  76. 

86 


NATURAL   PHILOSOPHY.  8? 

Lower  a  little  stretched  membrane  covered  with  sand  into 
an  organ-pipe  whose  front  is  glass,  as  shown  in  Figure  77, 
while  the  pipe  is  emitting  a  sound.  The  sand  will  be  seen 
to  be  agitated,  showing  that  the  air 
within  the  pipe  is  in  a  state  of  vibra- 
tion. 

98.  Fundamental  and  Har- 
monic Vibrations.  — -  By  fun- 
damental vibrations  we  mean 
the  vibrations  that  are  execut- 
ed   by   a    body   as   a   whole ; 
and    by   harmonic    vibrations 
those  which  are  executed  by 
the  parts,  or  segments,  of  the 
body.       Figure    78    shows    a 
string  vibrating   as    a   whole, 
and   in    two,   three,   and    four 
segments.     The  harmonic  vi- 
brations are  more  rapid  than 
the    fundamental    vibrations ; 
and  the  smaller  the  vibrating 
segments  the  quicker  are  the 
vibrations. 

Whenever  the  fundamental 
vibrations     of      a     body     are 

started,  some  of  the  harmonic  vibrations  are  almost 
certain  to  be  started  with  it  :  hence  the  molar  vibra- 
tions which  produce  a  sound  are  more  or  less  com- 
plicated. 

99.  Quality  of  Sound.  —  The  quality  of  a  sound 
depends   entirely  upon    the   character  of   the  vibra- 
tions which  produce  it ;  that  is,  on  the  number  and 


88        .  NATURAL   PHILOSOPHY. 

kind  of  the  harmonics  which  are  combined  with 'the 
fundamental  vibration.  A  rough,  irregular  sound  is 
called  a  noise ;  and  a  smooth  and  regular  one  is 
called  a  musical  sound. 

Every  change  in   the  number,  kind,  or  intensity 
of  the  harmonic  vibrations  present  causes  a  change 
in  the  quality  of  the  sound.     One  vowel  sound  or 
one  musical  tone  differs  from  another  simply  because 
of  some  difference  in  the  harmonic  vibrations  which 
produce  it.     Every  articulate  sound,  as  well  as  every 
musical  tone,  is  produced  by  a  particular  combina- 
tion of  harmonic  and 
fundamental      vibra- 
tions; and  whenever 
that  particular  kind 
of   vibration  is   pro- 
duced,    no      matter 
how,    the    particular 
sound   which    corre- 
sponds     to      it      is 
Fis-  78.  heard.     We  shall  see 

farther  on  that  it  is  possible  to  produce  articulate 
sounds  and  words  by  other  means  than  the  organs 
of  speech. 

100.  Loudness  and  Pitch  of  Sound.  —  The  loud- 
ness,  or  intensity,  of  sound  depends  upon  the  energy 
of  the  vibrations.  The  pitch  of  sound  depends  upon 
the  rapidity  of  the  vibrations.  Two  sounds  are  said 
to  be  in  unison  when  their  rates  of  vibration  are 
the  same ;  and  to  form  an  octave,  when  their  rates 
of  vibration  are  as  two  to  one. 


NATURAL  PHILOSOPHY.  89 

Illustrations  of  Pitch.  —  In  the  lowest  note  of  the  organ, 
there  are  sixteen  and  a  half  vibrations  a  second.  In  the  lowest 
note  of  the  piano  tnere  are  thirty-three  vibrations  a  second, 
and  in  the  highest  note  4224,  giving  a  range  of  seven  octaves. 
In  the  highest  note  ever  heard  in  an  orchestra,  there  are 
4752  vibrations  a  second.  This  note  is  given  by  the  piccolo 
flute.  In  the  shrillest  sounds  that  are  audible  there  are 
about  32,ot>o  vibrations  a  second,  the  upper  limit  of  audibility 
varying  with  different  persons.  The  voice  of  ordinary  chorus- 
singers  ranges  from  a  hundred  to  a  thousand  vibrations  a  sec- 
ond, and  the  extreme  limits  of  the  human  voice  are  fifty  and 
fifteen  hundred  vibrations  a  second. 

101.  Stringed    Instruments.  —  In    one    class    of 
musical  instruments  the  notes  are  produced  by  the 
transverse  vibrations  of  strings.     These  instruments 
are  called  stringed  instruments.     The  rate  at  which 
a  string  vibrates  depends  upon  its  length,  its  weight, 
and  its  tension.     The  shorter,  the  tightp,  and  the 
lighter,  a  string,  the  faster  it  vibrates.     Strings  may 
be  thrown  into  transverse  vibration  by  drawing  a 
rosined   bow   across   them,   as   in   the   case   of   the 
violin ;  or  by  plucking  them  with  the  finger,  as  in 
the  case  of  the  harp;  or  by  striking  them  with  a 
hammer,  as  in  the  case  of  the  piano. 

Illustrations.  —  In  the  piano  there  is  a  string  for  every 
note.  In  the  violin  and  similar  instruments,  several  notes  are 
obtained  from  the  same  string  by  fingering  it  so  as  to  change 
its  length  and  tension. 

1 02.  Wind  Instruments.  —  In  wind  instruments 
the  notes  are  produced  by  the  longitudinal  vibra- 
tions of  columns  of  air  enclosed  in  pipes.     The  rate 
of  vibration  depends  upon  the  length  of  the  column, 
and  upon  whether  the  pipe  is  opened  or  closed.     The 


90  NATURAL  PHILOSOPHY. 

shorter  a  column  of  air,  the  faster  it  vibrates ;  and 
the  air  in  an  open  tube  vibrates  twice  as  fast  as 
that  in  a  closed  pipe  of  the  same  length. 

The  air  in  the  pipe  of  a  wind  instrument  is 
thrown  into  vibration  sometimes  by  the  vibrations 
of  the  lips  when  the  air  is  blown  through  them,  as 
in  the  case  of  the  trumpet  ;  or  by  the  vibration  of 
a  spring  called  a  reed  when  the  air  is  blown  against 
it,  as  in  the  case  of  the  clarinet ;  or  by  the  flutter 
of  a  jet  of  air  when  blown  against  a  sharp  edge, 
as  in  the  case  of  the  flute. 

Illustrations.  —  In  an  organ  there  are  as  many  pipes  as 
notes,  only  one  note  being  obtained  from  each  pipe.  In  the 
case  of  the  flute  and  similar  wind  instruments,  several  notes 
are  obtained  from  one  pipe  by  opening  and  closing  the  holes 
at  the  side  of  the  pipe  so  as  to  alter  the  length  of  the  vibrat- 
ing column  of  air,  and  by  altering  the  strength  of  the  blast 
so  as  to  change  from  the  fundamental  note  of  the  pipe  to 
one  or  other  of  its  harmonics. 

QUESTIONS. 

I.  What  is  the  condition  of  every  sounding  body?  2.  In 
what  does  sound  originate  ?  3.  Give  the  illustration  of  the 
tuning-fork.  4.  Of  the  violin-string.  5.  Of  the  organ-pipe. 
6.  What  do  we  mean  by  fundamental  vibrations  ?  7.  By  har- 
monic vibrations  ?  8.  What  is  true  of  the  rate  of  these  different 
vibrations  ?  9.  Upon  what  does  the  quality  of  sound  depend  ? 
10.  What  is  the  difference  between  a  noise  and  a  musical 
sound?  ii.  What  will  cause  a  change  in  the  quality  of  sound? 
12.  Why  does  one  vowel  sound  or  one  musical  tone  differ  from 
another?  13.  By  what  is  every  articulate  sound  produced? 
14.  What  follows  when  that  particular  kind  of  vibration  is 
produced  by  any  means  whatever?  15.  Upon  what  does  the 
loudness  of  sound  depend?  16.  Upon  what  does  the  pitch 
of  sound  depend?  17.  When  are  two  sounds  said  to  be  in 


NATURAL  PHILOSOPHY.  91 

unison?  18.  To  form  an  octave?  19.  Give  illustrations  of 
range  of  pitch.  20.  How  are  musical  sounds  produced  in 
stringed  instruments  ?>  21.  Upon  what  does  the  rate  of  vibra- 
tion depend  ?  22.  How  are  the  strings  thrown  into  vibration  ? 
23.  How  do  we  obtain  the  different  notes  in  a  piano?  24.  In  a 
violin  ?  25.  How  are  musical  sounds  produced  in  wind  instru- 
ments ?  26.  Upon  what  does  the  rate  of  vibration  depend? 
27.  How  is  the  air  in  the  pipe  thrown  into  vibration  ?  28.  How 
are  the  different  notes  obtained  in  the  organ?  29.  In  the 
flute? 

CHAPTER    XV. 

PROPAGATION    OF   SOUND    AND    SYMPATHETIC 
VIBRATIONS. 

103.  Sound   is   Propagated  by  all  Elastic  Sub- 
stances. —  Sound  is  not  propagated  in  a  vacuum, 
but  is  propagated  by  all  elastic  substances,  whether 
solid,    liquid,    or   gaseous.     Sounds   are   propagated 
chiefly  by  the  air. 

Illustrations.  —  If  a  bell  is  hung  in  the  middle  of  a  glass 
receiver  from  which  the  air  has  been  ex- 
hausted, as  shown  in  Figure  79,  no  sound  is 
heard  when  the  bell  is  rung.  If  air,  hydro- 
gen, or  any  other  gas,  is  now  allowed  to  pass 
into  the  receiver,  the  sound  of  the  bell  is 
heard  again.  If  a  bell  is  put  under  water 
and  struck,  it  can  be  heard.  If  a  person 
puts  his  ear  close  to  the  rail  of  an  iron 
fence,  and  the  rail  is  struck  at  a  considera- 
ble distance,  he  hears  the  blow  twice.  The  blg-  79> 
first  sound  comes  through  the  rail;  the  second,  which  soon 
follows,  comes  through  the  air. 

104.  Sound   is   Propagated   by  Waves.  —  As    a 
sounding  body  moves  to  and  fro  in  the  air,  it  starts 


92 


NATURAL  PHILOSOPHY. 


a  series  of  waves  in  the  air,  in  the  same  way  that 
a  board,  when  moved  to  and  fro,  would  start  a 
series  of  waves  in  water.  These  sound-waves  trav- 
erse the  air,  spreading  in  every  direction  from  the 
vibrating  body.  As  they  beat  against  the  ear,  they 
awaken  in  us  a  sensation  of  sound.  The  propa- 
gation of  sound  by  means  of  waves  is  shown  in 
Figure  80. 

Were  sound-waves  visible,  they  would  be  seen  to 
differ  considerably  from  water-waves.     The  particles 


Fig.  80. 

would  be  seen  to  vibrate  to  and  fro  in  the  direc- 
tion in  which  the  wave  is  advancing ;  that  is,  longi- 
tudinally, not  transversely,  or  across  the  direction  in 
which  the  wave  is  advancing,  as  in  the  case  of 
water-waves.  Instead  -  of  being  alternately  raised 
above,  and  depressed  below,  the  general  level,  so  as 
to  form  crest  and  hollow,  as  in  the  case  of  water- 
waves,  the  molecules  in  sound-waves  are  alternately 
crowded  together  and  drawn  apart,  so  as  to  form 
compressed  and  rarefied  portions,  or  phases. 

105.   Form  of  Sound-Waves.  —  By  the  form    of 


NATURAL  PHILOSOPHY.  93 

a  water-wave  we  mean  the  outline  of  the  surface 
of  the  wave.  By  the  form  of  a  sound-wave  we 
mean  the  degree  of  condensation  or  rarefaction  in 
the  two  phases  of  the  wave.  Were  the  sound-wave 
visible,  we  should  see  that  there  was  a  special  form 
of  sound-wave  corresponding  to  each  variety  of 
vibration  of  the  sounding  body.  Each  musical  tone 
and  each  articulate  sound  would  be  seen  to  have 
its  own  wave-form,  which  would  differ  from  every 
other  wave-form.  We  should  see  that  sounds  of 
high  pitch  would  have  short  waves,  and  those  of 
low  pitch  long  waves. 

1 06.  The  Velocity  of  Sound.  —  Sound  travels 
through  the  air  at  the  rate  of  ten  hundred  and 
ninety  feet  a  second,  at  the  temperature  of  the  freez- 
ing-point. This  is  at  the  rate  of  about  a  mile  in 
five  seconds. 

The  velocity  of  sound  in  air  depends  somewhat 
upon  the  state  of  the  atmosphere.  -  Sound-waves 
travel  faster  with  the  wind  than  against  it ;  and  the 
higher  the  temperature  of  the  air,  the  greater  the 
velocity  of  sound  in  it. 

The  velocity  of  sound  in  water  is  about  forty- 
seven  hundred  feet  a  second,  and  its  velocity  in 
solids  is  still  greater. 

Illustrations.  —  When  you  see  a  person  chopping  at  a  dis- 
tance, you  can  always  see  him  strike  some  time  before  you 
hear  the  blow.  When  a  cannon  is  fired  at  night  a  mile  away 
from  us,  we  can  see  the  flash  about  five  seconds  before  we 
hear  the  report.  We  see  the  flash  of  lightning  before  we  hear 
the  thunder.  The  nearer  the  lightning,  the  shorter  is  the 
interval  between  the  flash  and  the  thunder. 


94  NATURAL  PHILOSOPHY. 

107.  The  Reflection  of  Sound,  rr-r  When    sound- 
waves meet  the  surface  of  a  new  medium,  they  are,, 
in  part,  thrown  back,  or  reflected.     In  this  reflection, 
as  in  all  cases  of  reflected  motion  (52),  the  angles  of 
incidence  and  reflection  are  equal  to  each  other. 

Illustrations. —  Echoes  are  produced  by  the  reflection  of 
sound.  In  order  to  get  an  echo,  we  must  have  a  reflecting 
surface  far  enough  away  to  give  an  appreciable  interval  be- 
tween the  direct  and  reflected  sounds.  When  the  surface  is 
less  than  a  hundred  feet  distant,  the  reflected  sound  blends 
with  the  direct  sound. 

The  reflecting  surface  has  often  such  a  shape  as  to  cause 
the  different  portions  of  the  reflected  wave  to  converge  to 
a  point,  and  so  to  intensify  the  reflected  sound. 

Multiple  echoes  may  be  produced  by  successive  reflections 
from  surfaces  at  different  distances  on  the  same  side,  or  by 
alternate  reflections  from  two  surfaces  on  opposite  sides.  In 
some  localities  a  pistol-shot  is  repeated  thirty  or  forty  times. 

108.  Sympathetic  Vibrations. — Whenever  sound- 
waves encounter  a  body  which  is  capable  of  vibrat- 
ing at  the  rate  at  which  the  waves  follow  each  other, 
they  throw  it  into  vibration.     Vibrations  started  in 
this   way,    by   the   pulsations    of   sound,   are   called 
sympathetic  vibrations. 

Each  wave,  as  it  meets  the  body,  gives  it  a  little 
push,  and  moves  it  forward  a  little  way.  The  body 
is  then  released,  and  flies  back  ;  and  the  next  wave 
meets  it  just  in  time  to  give  it  another  push  as 
the  body  is  ready  to  start  forward  again  of  itself. 
Each  wave  pushes  the  body  but  little ;  but  the 
pushes  are  so  timed,  that  each  moves  it  a  little 
farther  than  the  last,  until  the  body  is  made  to 
vibrate  strongly. 


NATURAL  PHILOSOPHY.  95 

When  the  body  cannot  vibrate  at  the  rate  at 
which  the  waves  succeed  each  other,  the  waves  will 
sometimes  push  the  body  in  the  direction  in  which 
it  is  moving  of  itself,  and  sometimes  in  the  oppo- 
site direction.  In  this  case,  one  push  will  neutral- 
ize the  effect  of  another  instead  of  augmenting  it. 
It  is  like  pushing  a  person  who  is  swinging.  A 
succession  of  pushes,  rightly  timed,  may  make  a 
heavy  person  swing  powerfully  ;  while  the  same 
pushes,  or  even  stronger  pushes,  wrongly  timed, 
would  not  only  fail  to  set  one  swinging,  but  stop 
one  who  was  already  swinging. 

Illustrations.  —  Take  two  tuning-forks  of  exactly  the  same 
pitch ;  cause  one  of  them  to  vibrate,  and  hold  it  near  the 
other  without  touching  it.  The  second  fork  will  soon  begin 
to  vibrate,  and  will  emit  a  distinctly  audible  sound  after  the 
first  has  been  stopped.  The  second  fork  will  not  be  started 
by  the  first  unless  the  two  are  of  exactly  the  same  pitch,  as 
may  be  shown  by  sticking  a  little  pellet  of  wax  to  the  prong 
of  one  of  the  forks,  so  as  to  diminish  its  rate  of  vibration. 

If  a  piano  is  opened,  and  one  of  the  keys  gently  depressed, 
so  as  to  raise  the  damper  without  striking  the  string  with  the 
hammer,  and  the  note  of  the  string  is  then  sung  over  the 
piano,  the  string  will  begin  to  vibrate,  and  will  emit  an  audible 
sound  for  a  little  time  after  the  voice  ceases.  It  is  only 
necessary  to  hit  the  pitch  of  a  string  accurately,  and  to  sus- 
tain the  note  sufficiently. 

If  a  vibrating  tuning-fork  is  held  at  the  end  of  a  tube  an 
fnch  and  a  half  or  two  inches  in  diameter,  the  sound  of  the 
fork  will  be  powerfully  reinforced,  if  the  tube  is  of  suitable 
length.  The  suitable  length  for  a  tube  open  at  both  ends  is 
one-half  of  the  length  of  the  wave  produced  by  the  fork.  A 
tube  closed  at  one  end  resounds  most  powerfully  when  its 
length  is  one-quarter  of  the  length  of  the  wave  produced  by 
the  fork.  The  column  of  air  in  the  tube  is  thrown  into  power- 


96  NATURAL  PHILOSOPHY. 

ful  sympathetic  vibrations  by  the  fork,  and  these  vibration 
greatly  augment  the  sound.  The  moment  the  fork  is  stopped, 
the  sympathetic  sound  ceases. 

109.  Sympathetic  Vibrations  of  Thin  Mem- 
branes.—  Thin  membranes,  when  stretched,  are 
very  readily  thrown  into  sympathetic  vibration ;  but 
their  vibrations  stop  promptly  when  the  exciting 
sound  ceases.  Owing  to  the  facility  with  which 
they  break  up  into  vibrating  segments,  they  respond 
readily  to  all  rates  of  vibration.  The  same  is  true 
of  thin  metallic  plates 


Fig.  81. 

no.  Edison's  Phonograph. —  Edison's  phono- 
graph,  which  is  shown  in  Figure  81,  consists  of  a 
cylinder  C,  and  of  a  mouth-piece  F.  An  enlarged 
view  of  the  mouth-piece  is  shown  in  Figure  82. 
At  the  bottom  of  the  conical  opening  of  the  mouth- 
piece is  a  thin  metallic  plate  A.  Under  this  plate 
is  a  point  P,  which  is  separated  from  the  metallic 
plate  by  a  piece  of  rubber  tube  x,  against  which 
it  is  held  by  the  spring  E.  The  cylinder  is  turned 
by  the  crank  D.  A  screw  cut  in  one  end  of  the 
axis  causes  the  cylinder  to  move  along  horizontally 
as  it  rotates.  A  shallow  spiral  groove  is  cut  in 
the  surface  of  the  cylinder  in  such  a  way  as  to  be 


NATURAL  PHILOSOPHY. 


97 


always  under  the  point  P  as  the  cylinder  is  turned. 
A  sheet  of  tin-foil  is  fastened  smoothly  on  the  sur- 
face of  the  cylinder.  The  point  P  presses  against  this 
tin-foil ;  and,  if  the  metallic  plate  is  not  vibrating, 
this  point  will  mark  a  spiral  line  of  uniform  depth 
on  the  tin-foil  as  the  cylin- 
der is  turned.  If  we  speak 
or  sing  into  the  mouth- 
piece,  the  plate  A 
is  thrown  into  sym- 
pathetic vibration  ; 
and  the  vibrations 
of  this  plate  are 
exactly  like  those 
which  produce  the 
articulate  sounds 
of  the  words  spo- 
ken. The  point  P 
follows  the 
centre  of  the 
plate  in  its 
vibration.  If 
the  cylinder  is  turned  while  one  is  speaking  into 
the  mouth-piece,  the  point  will  mark  a  line  on  the 
foil  of  varying  depth,  the  depth  of  the  indentations 
corresponding  exactly  to  the  vibrations  of  the  point 
and  of  the  plate.  In  this  way,  the  vibrations  of 
the  plate  are  registered  on  the  sheet  of  tin-foil. 
Now,  pull  back  the  mouth-piece,  set  the  cylinder 
back  to  the  starting-point,  replace  the  mouth-piece, 
and  again  turn  the  cylinder.  As  the  indentations 
of  the  tin-foil  pass  under  the  point,  they  compel  it 


Fig.  82. 


98  NATURAL  PHILOSOPHY. 

to  move  to  and  fro  exactly  as  it  did  in  producing 
them,  and  the  point,  in  turn,  compels  the  plate  to 
vibrate  exactly  as  it  did  at  first,  and  therefore  to 
repeat  the  words  that  were  spoken  to  it.  This  may 
be  repeated  several  times,  and  the  words  may  be 
distinctly  heard  by  all  in  the  room. 

QUESTIONS. 

i.  By  what  substances  is  sound  propagated?  2.  By  what  is 
it  chiefly  propagated?  3.  Give  an  illustration  to  show  that 
sound  is  not  propagated  in  a  vacuum.  4.  That  it  is  propa- 
gated by  any  gas.  5.  By  a  liquid.  6.  By  a  solid.  7.  In  what 
way  is  sound  propagated?  8.  How  are  these  waves  started? 
9.  How  do  the  vibrations  in  sound-waves  differ  from  those 
of  water-waves  ?  10.  What  are  the  two  phases  of  sound-waves  ? 
ii.  What  is  meant  by  the  form  of  a  water-wave?  12.  By  the 
form  of  a  sound-wave?  13.  What  is  true  of  the  wave-form 
for  each  musical  tone  and  for  each  articulate  sound  ?  14.  What 
waves  have  sounds  of  high  pitch  and  low  pitch?  15.  What  is 
the  velocity  of  sound  in  air?  16.  In  water?  17.  In  solids? 

18.  Give   illustrations   of  the   velocity   of   sound    in   the   air? 

19.  What  is  meant  by  the  reflection  of  sound?     20.  What  law 
of  reflected  motion  applies  here?     21.  Explain  the  production 
of  echoes.     22.  In  what  two  ways  may  multiple  echoes  be  pro- 
duced ?     23.  What  are   sympathetic   vibrations  ?     24.   Explain 
how  these  vibrations   are   produced.      25.  Why  are   no  vibra- 
tions started  in  a  body  which  is  not  capable  of  vibrating  at  the 
rate  at  which  the  waves  follow  each  other?     26.  Give  the  illus- 
tration of  sympathetic  vibrations  in  the  case  of  the  tuning-fork. 
27.  In  the  case  of  the  piano-string.     28.  In  the  case  of  tubes. 
29.  In  the   case  of  thin   membranes.     30.  Describe    Edison's 
phonograph.     31.  Describe  the  vibrations  of  the  plate  and  of 
the  point.     32.  Explain  how  the  vibrations  are  registered   on 
the  tin-foil.     33.  Explain  how  the  plate  may  again  be  made  to 
vibrate  so  as  to  repeat  the  words  that  have  been  spoken  to  it. 


V. 

HEAT. 


CHAPTER    XVI. 

NATURE   AND   TRANSMISSION   OF  HEAT. 

in.  Nature  of  Heat.  —  Heat  originates  in  the 
molecular  and  atomic  vibrations  of  bodies.  The 
atoms  and  molecules  of  bodies  are  in  a  state  of 
constant  agitation,  vibrating  to  and  fro  with  very 
great  rapidity  ;  and  the  hotter  the  body,  the  more 
lively  is  this  movement.  A  body  feels  hot  when 
we  touch  it,  because  its  molecules  and  atoms  are 
beating  with  such  rapidity  and  vigor  against  the 
skin.  Any  thing  that  will  increase  the  agitation 
of  the  molecules  of  a  body  will  make  it  hotter. 

Illustrations.  —  When  we  rub  a  match  against  a  rough  sur 
face,  the  friction  increases  the  agitation  of  the  molecules,  and 
so  heats  the  end  of  the  match  enough  to  light  the  phosphorus 
on  it.  A  brass  button  rubbed  against  the  sleeve  becomes  hot 
for  a  similar  reason.  Friction  always  develops  heat.  A  black- 
smith may  heat  an  iron  nail  red-hot  by  striking  it  vigorous 
and  rapid  blows  with  a  hammer.  Every  blow  upon  the  nail 
increases  the  agitation  of  its  molecules  and  atoms.  In  the 
burning  of  a  piece  of  coal,  the  atoms  of  oxygen  in  the  air 
and  those  of  the  coal  rush  together  with  inconceivable  rapidity, 

99 


100  NATURAL  PHILOSOPHY. 

and  as  they  dash  against  one  another  they  are  thrown  into 
intense  vibration :  hence  the  heat  developed  in  this  and  other 
cases  of  combustion. 

112.  Radiation  of  Heat.  —  As    the   atoms    of    a 
body  move  to  and  fro  in  the  ether  (18),  they  start 
waves  in  the  ether  in  the  same  manner  that  a  stick 
moved  rapidly  to  and  fro  in  water  will  start  waves 
in   the   water.     These   ethereal   waves   are   exceed- 
ingly minute,  from  thirty  to  sixty  thousand  of  them 
being  required  to   make  an  inch  when  placed  end 
to  end ;  and  they  traverse  the  ether  with  a  velocity 
which    would    carry    them    more  than    seven    times 
around  the  earth  in  a  second.     By  means  of  these 
waves    the    heat    of    a    body    becomes    distributed 
through  space.     This  method  of  distributing  heat  is 
called  radiation.     The  body  in  which  the  waves  are 
started  is  said  to  radiate  heat.     Rough  and  black- 
ened  surfaces   radiate  heat   better  than   bright  and 
polished  ones. 

Illustrations.  —  The  heat  which  we  feel  when  we  are  near 
a  stove  or  other  hot  body  comes  to  us  chiefly  by  radiation. 
The  heat  of  the  sun  comes  entirely  by  radiation.  Stoves  are 
better  radiators  for  having  black  and  rough  surfaces.  Water 
will  keep  hot  longer  in  a  bright  polished  teapot  than  in  a 
rough  iron  kettle. 

113.  Absorption  of  Heat.  —  When    the    ethereal 
waves  beat  against  the   atoms   of   a   second   body, 
they  throw  these  atoms   into  vibration,  or  else  in- 
crease their  agitation,  and  thus   communicate  heat 
to  them.     The  heat   taken   up  in   this  way  by  the 
atoms  of  a  body  is  said  to  be  absorbed  by  the  body. 
The  best  radiators  are  also  the  best  absorbers. 


NATURAL  PHILOSOPHY.  IOI 


114.  Conduction  of  Heat.  —  If  one  *eiiclj  of3  in 
iron  poker  is  placed  in  the  fire,  the  htat  will  /bvs; 
found  to  travel  'slowly  along  the  poker  till  its  far- 
ther end  finally  becomes  hot.  This  slow  trans- 
mission of  heat  through  a  body,  from  molecule  to 
molecule,  is  called  conduction.  The  metals  are  good 
conductors  of  heat  :  glass,  wood,  straw,  wool,  liquids, 
and  gases,  are  poor  conductors  of  heat. 

Illustrations.  —  One  end  of  a  glass  rod  may  be  held  in  the 
flame  of  a  spirit-lamp  till  it  is  heated  white-hot;  and  yet  an 
inch  away  from  the  red-hot  portion  the  glass  scarcely  feels 
warm.  The  soldering  irons  used  by  plumbers  are  provided 
with  wooden  handles,  to  keep  them  from  allowing  the  heat  to 
pass  to  the  hand.  A  thin  pane  of  glass  is  sufficient  to  keep 
the  heat  from  escap- 
ing from  a  room.  A 
covering  of  straw  is 
an  excellent  protec- 
tion to  plants,  be- 
cause it  conducts  the 
heat  away  from  them 
so  slowly.  A  piece  of  cold  iron  feels  much  colder  than  a 
fleece  of  wool  or  a  piece  of  flannel  at  the  same  temperature, 
because  it  conducts  the  heat  away  from  the  hand  so  rapidly. 
Hair  and  feathers  are  excellent  protection  for  animals  and 
birds,  because  they  serve  to  keep  the  same  layer  of  air  in 
contact  with  the  skin,  and  the  air  is  a  very  poor  conductor 
of  heat. 

Experiments.  —  The  different  conducting  powers  of  differ- 
ent solids  may  be  shown  by  the  following  experiment :  two 
rods,  of  different  materials,  are  placed  near  together,  end  to 
end,  as  shown  in  Figure  83.  Little  balls  are  stuck,  at  equal 
intervals,  to  these  rods  with  wax.  The  ends  of  both  rods  are 
then  exposed  to  the  same  source  of  heat.  As  the  heat  passes 
along  the  rods,  the  wax  is  melted,  and  the  balls  drop.  The 
rod  from  which  the  balls  drop  the  faster  is  the  better  con- 
ductor of  heat. 


102 


NATURAL  PHILOSOPHY. 


To  show  the  poor  conducting  power  of  water,  put  a  piece 
of  i^ewat:the  bottom  of  a  test-tube,  and  nearly  fill  the  tube 

with  water.  Place  a  lamp  at 
the  middle  of  the  tube,  as 
shown  in  Figure  84.  The 
water  will  boil  in  the  top  of 
the  tube,  while  the  ice  will 
not  melt  at  the  bottom. 

115.  Convection.  —  If 
heat  is  applied  at  the 
bottom  of  a  liquid  or 
gas,  the  portions  of  the 
fluid  in  contact  with  the 
heat  become  lighter,  and 
rise,  while  the  colder  an'd 
heavier  fluid  on  every 
side  .comes  around  to  the 

heat  to  take  the  place  of  the  portions  which  have 

passed  off.     In  this  way, 

currents   are   established, 

which  distribute  the  heat 

by  carrying  it  away  with 

them.      This    method    of 

distribution     of    heat    is 

called  convection,  and  the 

currents    by   which   it    is 

distributed  are  called  con- 
vection currents.  Liquids 

and     gases     are     heated 

mainly     by      convection. 

The   fluid   rises  over  the 

centre  of  the  heated  por-  Fig>  8s' 

tion,  flows  away  from  this  centre  in  every  direction 


NATURAL  PHILOSOPHY.  103 

above,  flows  down  on  all  sides  around  the  ascend- 
ing column,  and  flows  in  towards  the  source  of 
heat  from  ever/  side  below. 

Illustrations.  —  When  heat  is  applied  to  the  bottom  of  a 
glass  vessel  filled  with  water  which  holds  small  particles  in 
suspension,  the  currents  will  be  seen  to  flow  in  the  directions 
indicated  by  the  arrows  in  Figure  85. 

If  a  door  into  a  warm  room  is  left  a  little  ajar,  and  a 
lighted  candle  is  held  at  the  top,  middle,  and  bottom  of  the 
door,  the  flame  of  the  candle  will  be  seen  to  be  blown  outward 
at  the  top,  and  inward  at  the  bottom,  while  it  will  remain 
steady  at  the  middle.  The  air  attempts  to  escape  from  every 
side  at  the  top  of  a  heated  room,  and  to  enter  from  every  side 
at  the  bottom.  It  is  mainly  by  the  air  entering  and  escaping 
through  the  cracks  at  the  doors  and  windows  that  the  room 
becomes  ventilated. 

Winds  are  convection  currents  on  a  large  scale  in  the 
atmosphere.  Portions  of  the  atmosphere  become  excessively 
heated  by  contact  with  the  earth,  or  by  other  means,  and  so 
start  these  currents. 

QUESTIONS. 

i.  In  what  does  heat  originate  ?  2.  What  takes  place  when 
a  body  is  heated  ?  3.  Why  does  a  body  feel  hot  ?  4.  What 
will  make  a  body  hotter  ?  5.  Give  three  illustrations.  6. 
What  do  the  vibrating  atoms  start  in  the  ether  ?  7.  Describe 
these  waves.  8.  What  do  these  waves  do  ?  9.  What  do  we 
mean  by  the  radiation  of  heat  ?  10.  What  kind  of  surfaces 
are  good  radiators  ?  u.  Give  illustrations  of  the  radiation  of 
heat.  12.  How  do  bodies  absorb  heat?  13.  What  bodies  are 
the  best  absorbers?  14.  What  do  we  mean  by  the  conduction 
of  heat?  15.  Give  an  illustration.  16.  Name  some  substances 
which  are  good,  and  some  which  are  poor  conductors.  17. 
Why  have  soldering  irons  wooden  handles  ?  18.  Why  will  a 
thin  pane  of  glass  keep  out  the  cold?  19.  What  material 
makes  a  good  winter  covering  for  plants  ?  20.  Why?  21.  Why 


104  NATURAL  PHILOSOPHY. 

does  a  piece  of  iron  feel  colder  than  wool  ?  22.  Why  are 
feathers  and  fur  so  good  a  protection  against  cold  ?  23.  Give 
an  experiment  which  shows  the  unequal  conducting  power 
of  different  solids.  24.  Give  an  experiment  which  shows  the 
poor  conducting  power  of  liquids.  25.  What  is  meant  by  the- 
convection  of  heat?  26.  Describe  convection  currents,  and 
tell  how  they  are  started.  27.  Describe  the  convection  cur- 
rents formed  in  heating  water.  28.  Describe  the  convection 
currents  in  ,the  case  of  a  heated  room..  29.  Describe  an 
experiment  which  will  show  the  existence  of  these  currents. 
30.  What  are  the  winds?  31.  How  are  they  started? 


CHAPTER    XVII. 

THE    THREE    EFFECTS    OF   HEAT. 

116.  Expansion.  —  An  almost  universal  effect  of 
heat,  when  imparted  to  bodies,  is  to  cause  them  to 
become  larger,  or  to  expand  them.  Heat  expands 
bodies  by  causing  their  molecules  to  separate  from 
each  other.  Gases  expand  more  than  liquids,  and 
liquids  more  than  solids,  for  the  same  rise  of  tem- 
perature. Different  solids  and  liquids  expand  un- 
equally when  heated  equally,  but  all  gases  expand 
alike.  The  expansive  power  of  a  solid  when  heated 
is  almost  irresistible.  A  rod  of  iron  an  inch  square, 
when  heated  from  32°  to  212°,  exerts  an  expansive 
power  of  about  fifte'en  tons  in  the  direction  of  its' 
length. 

Illustrations.  —  In  all  structures  in  which  metals  are  em- 
ployed, the  parts  must  be  arranged  in  such  a  way  that  the 
expansion  shall  not  be  attended  with  evil  effects.  In  a  railway, 
the  rails  are  not  placed  in  contact,  but  left  a  little  apart  to 
allow  room  for  variation  of  length.  Iron  beams  employed  in 


NATURAL  PHILOSOPHY.  105 

building  must  have  their  ends  free  to  move  without  encoun- 
tering obstacles,  which  they  would  inevitably  overthrow. 

Experiments.  —  The  expansion  of  a  solid  when  heated  may 
be  illustrated  by  means  of  the  ring  and  ball  shown  in  Figure 
86.  When  cool,  the  ball  passes  readily  through  the  ring;  but 
when  heated  it  will  rest  upon  the  ring,  as  shown  in  the  figure, 
without  falling  through. 

The  expansion  of  a  liquid  when  heated  may  be  illustrated 
by  means  of  a  glass  bulb  with  a  projecting  tube,  as  shown  in 
Figure  87.  The  bulb  and  a  part  of  the  tube  are  rilled  with 
the  liquid  to  be  tried.  The  bulb  is  then  heated  by  immersing 


Fig.  86. 

it  in  hot  water.  At  first  the  liquid  falls  a  little  in  the  stem, 
owing  to  the  fact  that  the  bulb  itself  becomes  heated  and 
expands  before  the  liquid  in  it  begins  to  expand.  Soon,  how- 
ever, the  liquid  begins  to  rise  in  the  stem. 

The  expansion  of  a  gas  may  be  illustrated  by  means  of  a 
bulb  with  a  long  projecting  tube,  as  shown  in  Figure  88.  If 
we  heat  the  bulb  by  grasping  it  in  the  hand,  the  little  column, 
or  index,  of  mercury,  m,  is  seen  to  move  forward,  showing 
that  the  air  in  the  bulb  has  expanded.  The  index  of  mer- 
cury is  introduced  into  the  tube  in  the  first  place  by  heating 
the  bulb,  so  as  to  drive  out  some  of  the  air  by  expansion. 
A  little  mercury  is  then  dropped  into  the  cup  a,  and  the  bulb 
allowed  to  cool.  As  the  bulb  cools,  the  air  in  it  contracts, 


io6 


NATURAL  PHILOSOPHY. 


and  the  mercury  is  forced  into  the  tube  by  the  pressure  of 
the  external  air. 

117.  Irregular  Expansion  and  Contraction  of 
Water.  —  As  water  cools,  it  contracts,  like  every 
other  liquid,  until  it  reaches  a  temperature  of  about 
39°.  It  then  begins  to  expand, 
and  expands  slowly,  till  its  tem- 
perature reaches  32°.  It  then 
freezes,  and  expands  considera- 
bly in  freezing.  After  it  has 
frozen,  it  begins  to  contract 
again  as  its  temperature  falls. 
Were  the  temperature  of  ice 
raised,  it  would  expand  till  it 
reached  32°.  It  would  then 
contract  considerably  on  melt- 
ing, and  continue  to  contract 
slowly,  till  the  temperature  was 
about  39° ;  it  would  then  ex- 
pand again. 

Illustrations.  —  Since  water  is  most 
dense  at  a  temperature  of  about  39°, 
it  begins  to  grow  lighter,  and  to  rise 
to  the  surface  before  it  gets  cold 
enough  to  freeze.  The  consequence 
is  that  water  begins  to  freeze  at  the 
surface  instead  of  at  the  bottom ;  and 
the  layer  of  ice  at  the  surface,  being  a  poor  conductor  of 
heat,  prevents  the  water  from  freezing  to  any  great  depth, 
unless  the  cold  is  excessive  and  of  long  continuance.  It 
is  because  of  its  expansion  in  freezing,  that  water  is  liable 
to  burst  the  pipes  and  vessels  in  which  it  is  allowed  to 
freeze, 


Fig.  87. 


Fig. 


NATURAL  PHILOSOPHY. 


107 


1 1 8.  Change  of  State. — A  second  effect  of  heat 
is    to   change  the  state  of   a  body.     A   solid,  when 
heated  to  a  sufficient   temperature,   melts,   and  be- 
comes   a   liquid  ;   and   a  liquid,  when    heated    to    a 
sufficiently  high  temperature,  boils,  and  becomes  a 
gas  or  vapor.     The    temperature   at  which    a   solid 
melts   is    called    its    melting-point    or  fnsing-point ; 
and  the  tempera- 
ture   at    which    a 

liquid  boils  is 
called  its  boiling- 
point.  The  melt- 
ing-point of  ice  is 
32°,  and  the  boil- 
ing-point of  water 
is  212°. 

119.  Evapora- 
tion. —  A    liquid 
like  water  evapo- 
rates at    all    tem- 
peratures,        but 
more    rapidly    as 
the     temperature 
rises.    The  evapo- 
ration of  water  is 
mo-re    rapid  when 

the  air  is  dry  than  when  it  is  moist.  It  is  also 
more  rapid  on  a  windy  day  than  on  a  still  day.  At 
temperatures  below  the  boiling-point,  the  evapora- 
tion takes  place  only  at  the  surface  of  the  liquid ; 
while  at  the  boiling-point,  it  takes  place  through 
out  the  liquid. 


io8 


NATURAL  PHILOSOPHY. 


Illustrations,  — Water  which  is  exposed  to  the  air  in  a  dish 
gradually  disappears,  because  it  evaporates,  and  passes  into  the 
atmosphere  as  invisible  vapor.  Wet  surfaces  soon  dry,  owing 
to  the  evaporation  of  the  water  on  them.  Clothes  will  dry 
much  faster  on  a  windy  day  than  on  a  still  day,  because  the 
wind  promotes  the  evaporation.  A  wet  slate  will  dry  quicker 
if  we  fan  it,  or  swing  it  to  and  fro  in 
the  air.  A  barrel  of  water  will  evaporate 
much  quicker  when  sprinkled  on  the 
pavements  than  when  left  in  the  barrel, 
because  it  exposes  a  greater  surface  from 
which  evaporation  can  take  place. 

Experiment.  —  If  water  is  heated  in 
a  glass  flask,  the  vapor  will  at  first  rise 
only  from  the  surface  of  the  water ;  but 
it  will  be  seen  to  escape  faster  and 
faster  as  the  temperature  rises.  When 
the  boiling-point  is  reached,  bubbles  of 
.  vapor  will  be  seen  to  rise  through  the 
liquid,  as  shown  in  Figure  89,  and  to 
burst  on  reaching  the  surface.  The 
upper  part  of  the  flask  will  now  be  filled 
with  invisible  vapor,  which  condenses 
into  a  mist  as  it  comes  in  contact  with 
the  colder  air  at  the  mouth  of  the 
flask. 


120.   Rise    of   Temperature. — 

A    third    effect    of   the   commimi- 
Fi§-  90.  cation    of    heat    to   a   body   is    to 

cause  its  temperatttre  to  rise.  By  the  temperature 
of  a  body  we  mean  its  power  of  imparting  heat  to 
other  bodies.  The  greater  this  power  of  imparting 
heat,  the  higher  is  the  temperature  of  a  body.  A 
portion  'of  the  heat  is  employed  in  pushing  the 
molecules  into  new  positions.  It  is  this  portion 


NATURAL  PHILOSOPHY. 


109 


which  causes  expansion  and  change  of  state.  An- 
other portion  causes  the  atoms  and  molecules  to 
vibrate  with  greater  rapidity.  It  is  this  portion 
which  raises  the  temperature  of  the  body.  The 
temperature  of  a  body  is  inde- 
pendent of  the  amount  of  heat 
in  it. 

A  unit  of  heat  is  the  amount 
of  heat  required  to  raise  one 
pound  of  water  one  degree  in 
temperature ;  and  specific  heat 
is  the  amount  of  heat  required 
to  raise  one  pound  of  a  given 
substance  one  degree  in  tem- 
perature. Different  substances 
differ  greatly  in  their  specific 
heat. 

Illustrations. —  It  takes  about  ten 
times  as  much  heat  to  raise  a  pound 
of  water  one  degree  in  tempera- 
ture  as    a   pound   of    iron,    and 
over   thirty    times    as    much    as 
to    raise    a    pound    of    mercury 
one  degree. 

121.  Thermometers.  —    =1$ 

A    thermometer  is    an    in- 
strument    for     measuring 

temperature.  The  ordinary  thermometer  consists  of' 
a  glass  tube  of  very  fine  bore,  with  a  bulb  blown 
in  one  end  of  it.  The  bulb  and  a  part  of  the 
tube  are  filled  with  mercury.  As  the  'temperature 
of  the  bulb  rises,  the  mercury  expands,  and  rises 


no 


NATURAL  PHILOSOPHY. 


in  the  tube.     When  the  temperature  falls,  the  mer- 
cury contracts,  and  falls  in  the  tube. 


How  a  Thermometer  is  Filled.  —  The  opening  in  the  tube 
of  a  thermometer  is  too  fine  to  allow  mercury  to  be  poured  into 
it.  In  filling  the  instrument,  a  little  cup  is  formed  at  the 
open  end  of  the  stem,  and  filled  with  mercury,  as  shown  in 


.    NATURAL  PHILOSOPHY.  Ill 

Figure  90.  The  bulb  is  heated,  and  a  part  of  the  air  is  driven 
out  through  the  mercury  by  expansion.  The  bulb  is  then 
allowed  to  cool.  The  air  in  it  contracts,  and  some  of  the 
mercury  from  the  cup  falls  into  the  bulb.  The  bulb  is  again 
heated,  and  the  mercury  in  it  boiled  for  some  time.  The  vapor 
of  the  mercury  drives  all  the  air  out  of  the  bulb  and  tube. 
The  bulb  is  then  again  cooled,  the  vapor  in  it  condenses,  and 
the  mercury  from  the  cup  falls  into  the  tube  and  bulb,  and 
completely  fills  them.  The  bulb  is  now  heated  up  to  the  high- 
est temperature  the  thermometer  is  intended  to  measure,  so 
as  to  expel  a  part  of  the  mercury,  and  the  top  of  the  tube 
is  melted  off,  so  as  to  close  it.  As  the  bulb  cools  the  third 
time,  the  mercury  contracts,  and  leaves  an  empty  space  in 
the  upper  part  of  the  tube. 

To  obtain  the  freezing-point,  the  bulb  of  the  thermometer 
is  placed  in  melting  ice,  as  shown  in  Figure  91,  and  the  top 
of  the  column  of  mercury  is  marked  on  the  stem.  To  obtain 
the  boiling-point,  the  bulb  and  a  portion  of  the  stem  are 
immersed  in  steam  from  boiling  water,  as  shown  in  Figure  92. 

QUESTIONS. 

i.  What  is  the  first  effect  of  heat  on  bodies?  2.  What 
takes  place  in  expansion  ?  3.  What  is  said  of  the  expansion 
of  the  different  states  of  matter  ?  4.  Of  the  expansive  power 
of  iron  ?  5.  What  precaution  must  be  taken  when  metals 
are  used  in  structures?  6.  Give  illustrations.  7.  Give  an 
experiment  illustrating  the  expansion  of  solids.  8.  Of  liquids. 
9.  Of  gases.  10.  Describe  the  expansion  and  contraction  of 
water,  u.  What  changes  of  state  are  effected  by  heat?  12. 
What  do  we  mean  by  the  melting-point?  13.  By  the  boil- 
ing-point? 14.  What  is  the  melting-point  of  ice?  15.  The 
boiling-point  of  water?  16.  What  is  said  of  the  evaporation 
of  water?  17.  Give  illustrations.  18.  Describe  the  experi- 
ment of  boiling  water.  19.  What  is  a  third  effect  of  heat? 
20.  What  do  we  mean  by  temperature?  21.  In  what  two  ways 
is  the  heat  communicated  to  a  body  employed  ?  22.  What 
is  a  unit  of  heat?  23.  What  do  we  mean  by  specific  heat? 


112  NATURAL  PHILOSOPHY. 

24.  Give  illustrations  of  difference  of  specific  heat.  25.  What 
is  a  thermometer?  26.  Explain  how  it  shows  changes  of 
temperature.  27.  Describe  the  filling  of  the  thermometer. 
28.  Explain  how  the  freezing  and  boiling  points  are  found. 


CHAPTER    XVIII. 

LATENT    HEAT. 

122.  Latent  Heat.  —  The  heat  that  is  employed 
in  maintaining  the  temperature  of  a  body  is  called 
sensible  heat;    while    that    which    is    employed    in 
expanding  the    body,    or   in    changing   its   state,    is 
called  latent  heat.     Whenever  a  body  is  expanded, 
or  a  solid  is  melted,  or  a  liquid  is  evaporated,  the 
molecules    are    pushed   into    new   positions,   or   put 
into    positions    of   advantage   (56).     Latent   heat   is 
therefore   molecular  energy  of  position,   or  a  kind 
of  potential  energy.     Sensible  heat  is,,  on  the  other 
hand,    molecular   energy   of    motion,    or   a   kind    of 
kinetic  energy. 

123.  Heat  Consumed  in  Expansion.  —  Whenever 
a  portion   of   matter  expands,  heat   is  consumed,  or 
rendered  latent.     When  a  gas  expands  without  being 
heated,  a  portion  of  the  sensible  heat  in  it  is  con 
verted  into  latent  heat,  and  the  temperature  falls. 
The  heat   thus   consumed   is  called  the  latent  heat 
of  expansion.     When   the  gas  is  again  compressed, 
its  latent  heat  is  again  converted  into  sensible  heat, 
and  its  temperature  rises. 

Experiment.  —  Let   a   receiver   which   has    a    thermometer 
passing  into  it  through  the  top  be  placed  upon  the  plate  of 


NATURAL  PHILOSOPHY.  113 

an  air-pump.  If  we  now  work  the  pump,  the  air  in  the  receiver 
expands,  and  the  mercury  falls  in  the  thermometer.  The  fall 
of  the  mercury  shows  that  the  air  becomes  chilled  by  the 
expansion.  If  the  air  in  the  receiver  is  moist,  a  slight  cloud 
will  be  seen  to  form  in  the  receiver  soon  after  the  exhaustion 
begins.  The  chilling  of  the  air  by  expansion  causes  some 
of  its  moisture  to  condense. 

Illustrations.  —  Were  a  cubic  foot  of  air  suddenly  raised 
from  the  surface  of  the  earth  two  miles  into  the  atmos- 
phere, it  would  expand  very  much,  because  the  air  around  it 
would  exert  less  pressure  upon  it.  The  expansion,  in  this 
case,  would  cause  the  temperature  to  fall  about  35°.  One 
reason  why  it  is  so  cold  in  the  upper  regions  of  the  atmos- 
phere is  that  the  heated  air  which  rises  from  the  surface 
becomes  chilled 
by  expansion. 

The  chilling  of 
the  air  in  the 
above  case  would 
cause  most  of 
the  vapor  in  the 
air  to  condense, 
so  as  to  form  a 

cloud  or  rain.  Clouds  and  rain  are  caused  chiefly  by  the 
chilling  of  ascending  currents  of  air  because  of  their  expan- 
sion. Figure  93  shows  clouds  that  are  formed  on  the  top 
of  ascending  columns  of  air. 

124.   Heat   Consumed    in    Liquefaction.  —  Heat 

is  consumed,  or  rendered  latent,  whenever  a  solid 
is  liquefied.  The  heat  thus  consumed  is  called 
the  latent  heat  of  liquefaction,  or  of  the  liquid 
formed.  It  takes  143  units  of  heat  to  melt  a 
pound  of  ice  without  raising  the  temperature  at 
all.  The  latent  heat  of  water  is  therefore  143. 
When  a  liquid  solidifies,  its  latent  heat  again 
becomes  sensible. 


114  NATURAL  PHILOSOPHY. 

Experiments.  —  Hang  up  a  piece  of  ice  in  a  warm  room 
over  a  dish  which  will  catch  the  water  dripping  from  it.  If 
we  apply  the  bulb  of  a  thermometer  to  the  ice,  it  will  be  found 
to  maintain  a  temperature  of  32°  till  it  is  all  melted.  If  we 
allow  the  water  which  drips  from  the  ice  to  fall  upon  the 
bulb  of  a  thermometer,  the  temperature  of  the  liquid  will  also 
be  found  to  be  32°.  The  ice  is  all  the  time  receiving  heat, 
but  its  temperature  is  not  raised.  All  the  heat  is  used  in 
melting  the  ice,  and  is  therefore  rendered  latent. 

Fill  a  small  beaker-glass  half  full  of  pulverized  nitrate  of 
ammonia,  and  set  it  on  a  piece  of  wet  board.  Pour  into  it 
about  an  equal  bulk  of  water,  and  stir  the  mixture  with  the 
bulb  of  a  thermometer.  The  temperature  will  quickly  fall  to 
1 8°  or  20°  above  zero,  and  in  half  a  minute  the  beaker  will 
be  frozen  to  the  wet  board. 

Illustrations.  —  Ice-qream  is  frozen  by  means  of  a  mixture 
of  salt  and  ice.  The  salt  causes  some  of  the  ice  to  melt, 
and  the  heat  thus  consumed  lowers  the  temperature  of  the 
mixture  below  the  freezing-point  of  the  cream. 

In  the  spring  of  the  yfcar  a  large  amount  of  heat  is 
consumed  in  melting  the  snow  and  ice,  without  raising  the 
temperature :  this  retards  the  approach  of  hot  weather.  In 
the  fall  of  the  year  an  equally  large  amount  of  heat  is  given 
out  in  the  formation  of  the  snow  and  ice,  without  any  lowering 
of  the  temperature  :  this  retards  the  approach  of  cold  weather. 

125.  Heat  Consumed  in  Evaporation.  —  When- 
ever a  liquid  evaporates,  heat  is  consumed,  or  ren- 
dered latent.  This  heat  is  called  the  latent  heat 
of  evaporation,  or  of  the  vapor  formed.  It  takes 
nearly  1000  units  of  heat  to  convert  a  pound  of 
water  into  vapor :  hence  the  latent  heat  of  watery 
vapor  is  about  1000.  When  a  vapor  condenses,  its 
latent  heat  again  becomes  sensible. 

Illustrations.  —  Dip  the  bulb  of  a  thermometer  into  alcohol 
or  ether.  A  film  of  the  liquid  will  stick  to  the  bulb  when  it 


NATURAL  PHILOSOPHY.  115 

/s  withdrawn,  and  this  film  will  quickly  evaporate.  This 
evaporation  will  cool  the  bulb  several  degrees,  as  will  be  shown 
by  the  fall  of  the  mercury  in  the  stem. 

No  matter  how  hot  the  fire  is,  the  water  in  a  kettle  cannot 
be  heated  above  212°.  When  the  temperature  reaches  that 
point,  all  the  heat  received  by  the  water  is  used  in  converting 
it  into  vapor,  and  is  therefore  rendered  latent. 

Fanning  cools  a  person  chiefly  because  it  promotes  evapo- 
ration from  the  face. 

QUESTIONS. 

I.  What  is  meant  by  sensible  heat?  2.  By  latent  heat? 
3.  What  kind  of  energy  is  each  ?  4.  What  always  takes 
place  when  a  body  expands  ?  5.  What  do  we  call  this  heat  ? 
6.  What  is  true  of  the  temperature  of  a  gas  which  expands 
without  being  heated?  7.  Why?  8.  What  takes  place  when 
the  gas  is  again  compressed  ?  9.  Describe  an  experiment 
which  shows  the  consumption  of  heat  by  expansion.  10.  Ex- 
plain the  formation  of  the  cloud  which  is  sometimes  seen  in 
this  experiment.  11.  How  much  would  the  temperature  of  a 
cubic  foot  of  air  fall  if  it  were  suddenly  carried  up  two  miles 
into  the  atmosphere  ?  12.  Why  would  it  fall  ?  »  13.  Why  would 
a  cloud  be  formed  ?  14.  What  is  the  chief  cause  of  the  forma- 
tion of  clouds  and  rain?  15.  What  takes  place  when  a  solid 
is  liquefied  ?  16.  What  name  is  given  to  this  heat?  17.  What 
is  the  latent  heat  of  water?  18.  What  takes  place  when  a 
liquid  solidifies?  19.  Give  the  experiment  of  the  melting  of 
a  piece  of  ice.  20.  Of  the  nitrate  of  ammonia  and  water. 
21.  Explain  how  ice-cream  is  frozen.  22.  What  retards  the 
approach  of  warm  weather  in  the  spring,  and  of  cold  weather 
in  the  fall  ?  23.  What  takes  place  when  a  liquid  evaporates  ? 
24.  What  name  is  given  to  this  heat?  25.  What  is  the  latent 
heat  of  steam?  26.  Give  the  illustration  of  the  evaporation 
of  ether  and  alcohol.  27.  What  is  the  highest  temperature 
to  which  water  in  an  open  vessel  can  be  raised  ?  28.  Why  ? 
29.  What  is  one  reason  why  fanning  cools  the  face? 


VI. 

LIGHT. 


CHAPTER    XIX. 

NATURE   AND   TRANSMISSION    OF   LIGHT. 

126.  Nature     and    Transmission     of     Light.  — 
Light,  like  radiant  heat,  originates  in  the  vibrations 
of  the  atoms  of  a  body,   which  start   minute  waves 
in  the  etJier.     These  waves  are  in  every  way  similar 
to    those    of  *heat,   and  they  traverse  the  ether  at 
the  same  rate.     Light  is  therefore  a  kind  of  molecu- 
lar energy.     When    these    minute  waves    enter  the 
eye,  and  beat  upon   the   fibres  of   the  optic  nerve, 
they  awaken  the  sensation  of  light.     A  single  line 
of  light  is  called  a  ray,  and  a  collection  of  parallel 
rays  is   called  a  beam.     A  body  which   emits   light 
of  its  own  is  called  a  luminous  body.     A  body  like 
glass,  that  will  let  light  pass  through  it,  is  said  to 
be  transparent ;   and   one   like   iron,   which  will   not 
let  light  pass  through  it,  opaque. 

127.  Light    Moves    in    Straight   Lines.  —  Light 
passes    through    a    uniform     medium     in    straight 
lines. 

116 


NATURAL  PHILOSOPHY. 


117 


Ilhistrations.  —  Allow  a  beam  of  sunlight  to  enter  a  dark- 
ened room  through  a  hole  in  the  shutter.  Its  path  across 
the  room  is  seen  to  JDC  a  straight  line. 

When  the  sun  shines  into  a  room  through  a  small  opening, 
it  always  forms  a  round  spot  of  light  on  the  floor  or  wall,  as 
shown  in  Figure  94.  This  is  owing  to  the  iact  that  the  rays 
travel  in  straight  lines,  as  shown  in  the  figure.  It  will  be  seen 
that  the  rays  coming  from  the  various  parts  of  the  sun  cross 
each  other  on  passing  through  the  opening :  hence  the  picture 
on  the  floor  is  that  of  the  sun  inverted. 


Fig.  94- 

Allow  light  to  enter  a  darkened  room  through  a  small 
opening  in  a  shutter,  and  place  a  screen  near  the  opening.  An 
inverted  and  distinct  picture  of  the  objects  without  will  be 
formed  upon  the  screen,  as  shown  in  Figure  95.  This  pic- 
ture is  formed  by  the  rays  of  light  from  the  objects  without, 
which  pass  through  the  opening.  The  picture  is  inverted; 
because,  the  rays  of  light  being  straight,  those  which  come 


n8 


NATURAL  PHILOSOPHY. 


from  the  top  of  an  object  will  fall  lower  down  on  the  screen 
than  those  which  come  from  the  bottom  of  the  object.  The 
picture  is  clear ;  because,  the  opening  being  small,  only  the  rays 
from  one  part  of  the  object  can  fall  upon  any  one  part  of 
the  screen.  Were  the  opening  large,  the  rays  from  several 
parts  of  the  object  would  fall  upon  the  same  part  of  the 
screen,  and  the  picture  would  become  blurred ;  just  as  a 
painting  would  become  blurred  and  indistinct,  if  the  artist 
were  to  paint  several  parts  of  an  object  in  the  same  place. 


Fig.  95- 

When  any  opaque  body,  as  a  sphere  (Figure  96),  is  placed 
in  front  of  a  luminous  point,  it  cuts  off  the  light  from  the 
space  behind  it.  The  space  thus  deprived  of  light  is  called 
the  shadow  of  the  body.  This  shadow  lies  between  straight 
lines  which  proceed  from  the  point,  and  just  graze  the  edge 
of  the  ball.  The  shadow  is  due  to  the  fact  that  light  moves 
in  straight  lines.  A  line  drawn  through  the  centre  of  the 
shadow,  in  the  direction  of  its  length,  is  called  the  axis  of 
the  shadow.  If  a  screen  is  held  at  right  angles  to  the  axis 


NATURAL  PHILOSOPHY.  1 19 

of  the  shadow,  the  outline  of  the  shadow  thrown  upon  it  will 
be  seen  to  be  the  same  as  that  of  the  object  which  cast  the 
shadow. 

128.  Reflection.  —  When  a  ray  of  light  falls 
upon  a  smooth  surface,  it  is  in  part  thrown  off, 
or  reflected.  The  ray  which  falls  upon  the  surface 
is  called  the  incident  ray,  and  the  ray  which  is 
thrown  back  from  the  surface  the  reflected  ray. 
The  angle  between  the  incident  ray  and  a  perpen- 


dicular drawn  to  the  surface  at  the  point  at  which 
the  ray  strikes  it,  is  called  the  angle  of  incidence ; 
and  the  angle  between  the  reflected  ray  and  this 
perpendicular,  the  angle  of  reflection.  In  the  reflec- 
tion of  light,  the  angles  of  incidence  and  of  reflection 
are  always  equal  to  each  other  (152). 

Illustration.  —  Place  a  small  reflector  upon  a  table  in  a 
darkened  room,  so  that  a  beam  of  sunlight  admitted  through 
a  hole  in  the  shutter  may  fall  upon  it,  as  shown  in  Figure  97. 
The  beam  will  be  seen  to  be  reflected  in  such  a  way  that  the 
angles  of  incidence  and  of  reflection  will  be  equal.  Tilt  the 
reflector  so  as  to  change  the  angle  of  incidence,  and  the  angle 
of  reflection  will  change  equally. 


120  NATURAL  PHILOSOPHY. 

129.   Refraction.  —  When   a   ray  of    light  passes 


Ficr.  97. 

obliquely   into    a    denser   medium,    it    is    bent,    or 


Fig.  98. 

refracted,  towards  \\  perpendicular  to  the  surface  of 


NATURAL  PHILOSOPHY. 


121 


the  medium,  at  the  point  where  the  ray  strikes 
the  medium ;  and,  when  the  ray  passes  into  a  rarer 
medium,  the  ray  is  refracted  in  the  opposite  direc- 
tion. 

Illustrations.  —  Allow  a  beam  of  light,  entering  by  the 
shutter  into  a  darkened  room,  to  fall  upon  the  surface  of 
water  contained  in  a  glass  tank,  as  shown  in  Figure  98.  The 
beam  will  be  seen  to  be  bent  as  it  enters  the  water.  A  little 


Fig.  99. 


milk  added  to  the  water  will  make  the  path  of  the  ray  in  the 
water  much  more  distinct. 

A  fish  or  other  object  in  water  will  be  seen  a  little  above 
its  real  position,  as  shown  in  Figure  99.  The  light  which 
comes  from  the  fish  is  bent,  on  passing  into  the  air,  away 
from  a  perpendicular  to  the  surface  of  the  water. 

For  a  similar  reason  a  stick  held  obliquely  in  water 
appears  bent  upwards,  as  shown  in  Figure  100. 

130.  Dispersion.  —  When  a  ray  of  white  light 
passes  through  a  prism,  it  is  separated  into  seven 


122 


NATURAL  PHILOSOPHY. 


different  colored  rays,  because  these  different  rays 
are  bent  unequally.  The  red  is  bent  the  least,  and 
the  violet  most.  The  seven  colors  obtained  are  red, 
orangey  yellow,  green,  blue,  indigo,  and  violet.  This 

separation  of  the 
ray  into  its  com- 
ponent colors  is 
called  dispersion. 
The  seven  colors 
thus  obtained  are 
called  the  pris- 
matic colors,  and 
the  colored  band 
formed  on  the 
Fig- 10°-  screen  by  the  rays 

after  dispersion  is  called  the  spectrum.  The  color 
of  a  ray  is  due  to  the  length  of  its  ethereal 
waves.  These  waves  are  longest  in  red  light,  and 


Fig.  101. 

shortest    in   violet.     Refrangibility   in    light    corre- 
sponds to  pitch  in  sound. 

Illustrations.  —  Place  a  glass  prism  GEF  (Figure  101)  in 
the  path  of  a  beam  of  light,  A  B,  admitted  into  a  darkened 
room  through  a  narrow  hole  in  the  shutter,  SS'.  The  beam 


NATURAL  PHILOSOPHY. 


123 


will  be  both  refracted  and  dispersed,  as  shown  in  the  figure. 
There  will  appear  on  the  wall  a  long  colored  band,  red  at 
the  bottom  and  violet  at  the  top. 

131.  Diffusion.  —  A  portion  of  the  light  which 
falls  upon  the  surface  of  a  body  is  scattered  irregu- 
larly in  all  directions.  The  light  thus  irregularly 


Fig.    102. 

reflected  is  said  to  be  diffused.  It  is  by  the  light 
which  they  diffuse  that  we  are  enabled  to  see  the 
surface  of  non-luminous  bodies. 

Illustrations.  —  Hold  a  small  reflector  in  the  path  of  a 
beam  of  light  admitted  into  a  darkened  room,  so  as  to  reflect 
it  into  a  glass  vessel  (Figure  102)  through  a  narrow  opening 
in  a  card  laid  on  the  top  of  the  vessel.  If  the  vessel  is 
empty,  the  beam  of  light  is  invisible  in  the  vessel  If  the 


124  NATURAL  PHILOSOPHY. 

vessel  is  filled  with  smoke,  the  path  of  the  beam  becomes 
very  bright,  owing  to  the  light  diffused  by  the  particles  of 
the  smoke  which  lie  in  the  path  of  the  beam.  Reflect  the 
beam  of  light  into  a  goblet  of  water  to  which  has  been  added 
a  teaspoonful  of  milk.  The  liquid  will  shine  like  a  lamp  on 
account  of  the  light  diffused  by  the  particles  of  milk  suspended 
in  the  water. 

132.  Absorption.  —  A  portion  of  the  light  which 
falls  upon  a  body  is  taken  up  and  retained  by  the 
body.  This  light  is  said  to  be  absorbed.  A  body 
which  absorbs  some  of  the  colors  in  preference  to 
others  will  diffuse  light  in  which  these  absorbed 
colors  are  wanting,  or  deficient.  Such  light  will 
appear  colored,  its  hue  depending  upon  the  kind 
of  light  which  has  been  absorbed.  The  color  of  a 
non-luminous  body  is  obtained  from  the  light  which 
falls  upon  it,  the  hue  being  produced  by  the  extinc- 
tion of  some  of  the  component  colors  by  absorp- 
tion. A  body  has  no  color  in  the  dark.  A  rose 
is  red,  because  it  absorbs  all  the  rays  but  the  red ; 
and  a  violet  is  violet,  because  it  absorbs  all  the 
rays  but  those  which  make  violet. 

QUESTIONS. 

i.  In  what  does  light  originate  ?  2.  How  is  it  transmitted? 
3.  How  is  a  sensation  of  light  awakened  ?  4.  What  is  light  ? 
5.  What  is  meant  by  a  ray  of  light?  6.  By  a  beam  of 
light?  7.  In  what  paths  do  rays  of  light  move  in  a  uniform 
medium?  8.  Give  the  illustration  of  a  beam  of  sunlight  in 
a  darkened  room.  9.  Of  the  spot  painted  by  the  sun  on 
shining  through  a  small  opening.  10.  Of  images  formed  by 
small  openings.  11.  Why  is  such  an  image  clear?  12.  Why 
is  it  inverted?  13.  Give  the  illustration  of  the  formation  of 
shadows.  14.  What  is  meant  by  the  axis  of  a  shadow  ? 


NATURAL  PHILOSOPHY.  12$ 

15.  What  is  the  outline  of  a  shadow  cast  upon  a  screen  per- 
pendicular to  the  axis?  16.  Why?  17.  What  is  meant  by 
reflection  of  light?'  18.  By  the  incident  ray?  19.  By  the 
reflected  ray?  20.  By  the  angle  of  incidence?  21.  By  the 
angle  of  reflection?  22.  What  is  the  law  of  reflection?  23. 
Give  an  illustration  of  reflection.  24.  What  is  meant  by  refrac- 
tion of  light?  25.  Give  the  illustration  of  refraction  in  the 
tank  of  water.  26.  In  the  case  of  the  fish  in  water.  27.  Of 
a  stick  in  water.  28.  What  is  meant  by  the  dispersion  of 
light?  29.  Name  the  prismatic  colors.  30.  What  is  meant 
by  the  spectrum?  31.  Upon  what  does  the  color  of  a  ray 
depend  ?  32.  To  what  does  refrangibility  in  light  correspond  ? 
33.  What  is  meant  by  diffusion  of  light  ?  34.  Give  the  illustra- 
tion of  the  jar  of  smoke.  35.  Of  the  glass  of  milk  and  water. 
36.  What  is  meant  by  absorption  of  light?  37.  What  non- 
luminous  bodies  are  colored?  38.  To  what  is  the  hue  of 
the  color  due?  39.  Whence  do  bodies  obtain  their  color? 
40.  Why  do  different  bodies  have  different  colors? 


CHAPTER    XX. 

MIRRORS. 

133.  Kinds    of    Mirrors. —  A    reflector   is    often 
called  a  mirror.     Mirrors  are  sometimes  pieces  of 
polished  metal,  sometimes  pieces  of  glass  coated  in 
front    with   polished   silver,    and   sometimes   pieces 
of  glass  coated  on  the  back  with  a  mixture  of  tin 
and  mercury,  as  in  the  case  of  the  ordinary  looking- 
glass.     Mirrors  with  a  flat  surface  are  called  plane 
mirrors  ;  those  with  a  concave  surface,  concave  mir- 
rors ;    and    those    with    a    convex    surface,   convex 
mirrors. 

134.  The  Visual  Angle.  —  The    visual  angle   of 
an   object,  or  of   any  portion   of   an  object,  is  the 


126  NATURAL  PHILOSOPHY. 

angle  subtended  by  the  object,  or  by  that  portion 
of  it ;  that  is  to  say,  it  is  the  angle  formed  by 
lines  drawn  from  the  extremities  of  the  object,  or 
the  portion  of  it  which  we  are  considering,  to  the 


Fig.  103. 

eye,  as  shown  in  Figure  103.  The  visual  angle 
might  be  defined  as  the  angle  formed  by  rays 
coming  from  the  extremities  of  the  object  to  the 
eye.  The  nearer  an  object,  the  larger  its  visual 
angle;  and,  the  larger  an  object  (the  distance  being 


Fig.  104. 

the  same),  the  larger  the  visual  angle.  Any  thing 
that  increases  the  angle  under  which  it  is  seen 
enlarges,  or  magnifies,  the  object ;  and  any  thing 
which  lessens  the  visual  angle  makes  the  object 
appear  smaller. 


NATURAL  PHILOSOPHY, 


127 


135.  Reflection  from  Plane  Mirrors.  —  When 
light  is  reflected  from  a  plane  surface,  neither  the 
divergency  of  'the  rays  nor  the  visual  angle  is 
changed.  Every  portion  of  an  object  seen  reflected 
in  a  plane  mirror  appears  to  be  just  as  far  behind 
the  mirror  as  it  really  is  in  front  of  it.  The  reflec- 
tion o£  an  object  in  a  mirror  is  called  an  image. 


Fig.  105. 

In  a  plane  mirror  the  image  appears  erect   and  of 
the  size  pf  the  object. 

Illustrations.  —  Figure  104  shows  that  the  rays  are  just  as 
divergent  after  reflection  as  they  were  before  reflection.  The 
object  appears  to  be  behind  the  mirror,  because  we  always  see 
an  object  in  the  direction  which  the  rays  coming  from  it  have 
on  entering  the  eye. 

Figure  105  shows  that  the  visual  angle  is  not  changed  by 
reflection  from  a  plane  mirror:  hence  the  image  appears  of 
the  same  size  as  the  object. 


128  NATURAL  PHILOSOPHY. 

Objects   seen    reflected    in    a    horizontal   mirror 
appear  inverted,   because   each   part   of   the   object 


Fig.  106. 

will  appear  just  as  far  below  the  mirror  as  it    is 
really   above   it  :    hence   the    part   which    is    really 

uppermost  will 
appear  lowest 
in  the  reflec- 
tion. 


Illustration.  — 
Figur^  1 06  repre- 
Fig-  I07'  sents   the   reflec- 

tion of  buildings  and  other  objects  in  water,  and  shows  why 
they  appear  inverted. 

136.  Reflection  from  Concave  Mirrors. — When 
light  is  reflected  from  a  concave  surface,  the  rays 
are  made  either  to  approach  each  other,  or  else  to 
separate  from  each  other  less  rapidly  than  they  did 


NATURAL   PHILOSOPHY, 


1 20 


before  reflection  ;  that  is  to  say,  the  rays  are  made 
either  convergent  or  less  divergent. 

Parallel  rays  with  a  concave  mirror  are  made  to 
converge,  as  shown  in  Figure  107.  The  point  F, 
to  which  they  are  made  to  converge,  is  called  the 
principal  focus  of  the  mirror. 


Fig.  ioS. 

Illustration.  —  Were  the  sun's  rays  allowed  to  fall  upon  a 
concave  mirror,  they  would  be  made  to  converge  to  a  focus, 
as  shown  in  Figure  108. 

Rays  diverging  from  a  point  beyond  tJie  principal 
focus  of  a  concave  mirror  will  be  made  to  converge 
to  a  point  also  beyond  the  principal  focus.  The 
point  to  which  they  are  made  to  converge  is  called 
a  conjugate  focus. 

Illustrations.  —  Rays   diverging   frpm    the  point  L  (Figure 


130 


NATURAL   PHILOSOPHY. 


109)  will  be  made  to  converge  to  /.  Rays  diverging  from  the 
point  /  would  be  made  to  converge  to  L.  Each  of  these 
points  is  the  conjugate  focus  of  the  other. 

Rays  diverging  from  a  point  within  the  principal 
focus  of  a  concave  mirror  will  be  made  to  be  less 


divergent  after  reflection,  as  shown  in  Figure- no. 
The  point  /,  from  which  they  appear  to  diverge 
after  reflection,  is  called  a  virtual  focus. 

137.   Images    formed    by    Concave    Mirrors.  — 
Whenever  a  concave  mirror  makes   the  rays  from 


Fig.  no. 

an  object  convergent  by  reflection,  it  forms  an 
inverted  image  of  the  object.  The  size  of  this 
image  depends  upon  its  distance  from  the  mirror 
as  compared  with  that  of  the  object.  Whichever 
is  nearer  the  mirror  is  the  smaller. 


NATURAL  PHILOSOPHY.  131 

Illustration.  —  The  concave  mirror  shown  in  Figure  1 1 1 
forms  an  inverted  image  of  the  distant  church,  which  is 
received  on  a  sheet  of  white  paper  held  in  the  focus  of  the 
mirror. 

Whenever  a  concave  mirror  makes  the  rays 
coming  from  an  object  less  divergent,  it  enlarges 
the  visual  angle  of  the  object,  and  so  magnifies  it, 


as  shown  in  Figure  112.  In  this  case  the  object 
must  be  between  the  principal  focus  and  the 
mirror ;  and  the  image,  which  will  be  erect,  will 
be  seen  in  the  mirror,  as  in  the  case  of  the  plane 
mirror. 

138.  Reflection  from  Convex  Mirrors.  —  When 
light  is  reflected  from  a  convex  mirror,  the  rays 
are  made  more  divergent,  and  the  visual  angle  is 


132 


NATURAL  PHILOSOPHY. 


Fig.  112. 


made    smaller.     Such     mirrors    form    erect    images 


Fig.  113. 

which    are   smaller   than    the    object,    as    shown    in 
Figure   113. 

QUESTIONS. 

i.  What  is  meant  by  a  mirror?  2.  Of  what  materials  are 
mirrors  usually  made?  3.  Name  the  three  forms  of  mirrors. 
4.  What  is  meant  by  the  visual  angle  ?  5.  Upon  what  does 
the  size  of  the  visual  angle  depend?  6.  What  is  the  effect 
produce"!  upon  the  appearance  of  the  object  by  a  change  in 


NATURAL  PHILOSOPHY.  133 

the  visual  angle  ?  7.  What  effect  has  reflection  from  a  plane 
mirror  upon  the  divergency  of  the  rays?  8.  Upon  the  visual 
angle  ?  9.  What  name  do  we  give  to  the  reflection  of  an 
object  in  a  mirror?  10.  What  kind  of  images  do  plane  mirrors 
give?  n.  Why  do  they  appear  to  be  behind  the  mirror?  12. 
Why  do  obiects  reflected  in  water  appear  inverted?  13.  What 
effect  has  reflection  from  a  concave  mirror  upon  the  rays? 

14.  What  is  meant  by  the  principal  focus  of  a  concave  mirror? 

15.  Where  must  a  point  be  situated  in  order  to  have  the  rays 
diverging  from  it  made  convergent  by  reflection?     16.  What 
do  we  mean  by  a  conjugate  focus?     17.  Where  must  a  point 
be  situated  in  order  that  the  rays  diverging  from  it  may  be 
made  less  divergent  by  reflection?     18.  What  do  we  mean  by 
a  virtual  focus  ?     19.  When  do  concave  mirrors  form  inverted 
images?     20.  What  is  true  of  the  size  of  these  images?     21. 
When   do  concave  mirrors  form    erect  images?     22.  What  is 
true  of  the  size  of  these  images  ?     23.  Why  ?     24.  What  effect 
has  reflection  from  a  convex  mirror  upon  the  divergency  of 
the    rays  ?     25.  Upon    the   visual   angle  ?     26.  What  kind   of 
images   are   formed    by   such    mirrors  ?     27.  Why   are    these 
images  smaller  than  the  object? 


CHAPTER    XXI. 

LENSES. 

139.  Kinds  of  Lenses.  —  A  lens  is  made  of  glass 
or  other  transparent  substance.  It  has  at  least 
one  curved  surface,  and  it  has  usually  a  circular 
outline.  A  front  and  a  side  view  of  a  lens  are  given 
in  Figure  114.  When  a  lens  has  one  or  more  con- 
vex surfaces,  it  is  called  a  convex  lens;  and,  when 
it  has  one  or  more  concave  surfaces,  a  concave  lens. 
If  a  lens  has  both  a  convex  and  a  concave  surface, 
it  is  called  a  convex  lens  when  the  convex  surface 


134 


NATURAL  PHILOSOPHY. 


curves  more  than  the  other,  and  a  concave  lens  when 
the  concave  surface  curves  the  more.  There  are  three 
forms  of  lenses  of  each  class.  The  six  forms  are 
shown  in  section  in  Figure  115.  The  first  is  called 


Fig.  114. 

a  double-convex  lens,  the  second  a  plano-convex,  the 
third  a  meniscus,  the  fourth  a  double-concave,  the 
fifth  a  plano-concave,  and  the  sixth  a  convexo- 
concave. 

140.   Refraction     by    Convex     Lenses.  --  When 
light   is    refracted    by  a   convex   lens,  the    rays    are 

rc  N  o 


Fig.  115- 


made  either  to  approach  each  other,  or  else  to  sepa- 
rate from  each  other  less  rapidly :  in  other  words, 
they  become  either  convergent  or  else  less  divergent. 


NATURAL   PHILOSOPHY. 


135 


Convex  lenses  produce  the  same  effects  by  refrac- 
tion that  concave  mirrors  do  by  reflection. 

Parallel  rays  with  a  convex  lens  are  made  con-. 
vergent,    as    shown    in    Figure    116.     The    point    F, 


Fig.  116. 


towards  which  they  are  made  to  converge,  is  called 
the  principal  focus  of  the  lens. 

Illustration.  —  Figure  117  shows  the  concentration  of   the 
sun's  rays  by  means  of  a  convex  lens.     The  cannon  is  placed 


Fig.  117. 

in  such  a  position  that  the  point  of  concentration  of  the  rays 
will  fall  in  the  right  place  to  ignite  the  powder  when  the  sun 
is  on  the  meridian. 


136  NATURAL  PHILOSOPHY. 

Rays  diverging  from  a  point  beyond  the  principal 
focus  are  made  to  converge  by  the  refraction  of  the 
lens,  as  shown  in  Figure  118.  The  point  /,  to  which 


Fig.  118. 

they  are  made  to  converge,  is  called  the  conjugate 
focus.  The  nearer  the  point  of  divergence  to  the 
principal  focus,  the  more  remote  its  conjugate  focus. 


Fig.  119. 

Rays  diverging  from  a  point  at  the  principal 
focus  become  parallel  after  refraction,  as  shown 
in  Figure  119. 


NATURAL  PHILOSOPHY.  137 

Rays  diverging  from  a  point  within  the  principal 


Fig.  120. 


focus  become  less  divergent  after  refraction  by  the 
lens,  and  have  a  virtual  focus  at  /  (Figure  1 19). 


Fig.  121. 


138  NATURAL  PHILOSOPHY. 

141.  Images  formed  by  Convex  Lenses.  —  Con- 


Fig.  122. 


vex  lenses   form  inverted  images  of   objects  which 


Fig.  123. 

are   beyond  their  principal  foci,  as    shown    in    Fig- 
ure 1 20. 


NATURAL  PHILOSOPHY.  139 

When  the  object  is  farther  from  the  lens  than  the 
image  is,  the  image  is  smaller  than  the  object,  as 
shown  in  Figure  121. 

When    the    object    is    nearer  the    lens    than    the 


Fig.  124. 

image   is,   the   image   is   larger  than   the   object,   as 
shown  in  Figure   122. 

When  the  object  is  placed  within  the  principal 
focus  of  a  convex  lens,  an  enlarged  image  of  it  is 
obtained,  as  shown  in  Figure  123.  ab  is  the  object, 


Fig.  125. 

and   AB   the   image.     It   is  enlarged,  because   the 
visual  angle  is  increased  by  the  refraction. 

142.   Refraction    by    Concave    Lenses.  —  When 
light  passes  through  a  concave  lens,  the  rays   are 


140 


NATURAL  PHILOSOPHY. 


made    more  divergent  by   refraction,    as   shown   in 
Figure  124. 

A  concave  lens  forms    an  erect  image,  which  is 
smaller  than  the  object,  as  shown   in  Figure   125. 


Fig.  126. 


AB  is  the  object,  and  ab  its  image.  The  image 
is  smaller  than  the  object,  because  the  refraction 
makes  the  visual  angle  smaller. 

143.  The  Camera  Obscura.  — If  light  is  admitted 


NATURAL  PHILOSOPHY. 


141 


through  an  opening  into  a  darkened  room,  and 
then  reflected  through  a  convex  lens,  as  shown  in 
Figure  126,  a  picture  of  the  outside  view  will  be 

C 


Fig.  127. 

formed  upon  a  screen  placed  in  a  proper  position 
to  receive  it.  Such  a  darkened  room  is  called  a 
camera  obscura. 

Illustrations.  —  Figure    127   shows   in   section   the    camera 
used  by  photographers.     BC  is  the  dark  chamber,  LL'  are 


Fig.  128. 

the  lenses  through  which  the  light  passes,  and  E  is  the  screen 
for  receiving  the  picture.     When  a  person   sits   in  front   of 


142  NATURAL  PHILOSOPHY. 

these  lenses,  a  small   inverted  picture  of  him  is  formed  on 
the  screen. 

The  eye  is  a  small  camera  obscura.  The  parts  of  the  eye 
are  shown  in  Figure  128.  A  is  the  cornea  (the  transparent 
convex  covering  of  the  front  of  the  eye),  and  E  the  crystalline 
lens,  which  is  a  double-convex  lens.  These  correspond  to  the 
lenses  in  the  photographer's  camera.  D  is  the  iris,  a  sort  of 


Fig.  129. 

curtain  before  the  crystalline  lens,  giving  the  color  to  the 
eye ;  C  is  the  pupil,  or  opening  through  the  iris  by  which  the 
light  is  admitted ;  K  is  the  retina,  which  contains  the  fibres 
of  the  optic  nerve  on  the  back  of  the  eye.  This  answers  to 
the  screen  in  the  camera.  Figure  129  illustrates  the  formation 
of  the  image  on  the  retina  by  the  action  of  the  lenses  of 
the  jsye. 

QUESTIONS. 

I.  What  is  a  lens  ?  2.  What  are  the  two  classes  of  lenses? 
3.  Name  the  three  lenses  of  each  class,  and  tell  what  surfaces 
each  has.  4.  What  effects  do  convex  lenses  produce  upon  a 
ray  of 'light  by  refraction?  5.  What  is  meant  by  the  principal 
focus  of  a  convex  lens  ?  6.  By  a  conjugate  focus  ?  7.  Upon 
what  does  the  distance  of  a  conjugate  focus  from  a  lens 
depend?  8.  What  rays  have  a  virtual  focus  with  a  convex 
lens  ?  9.  When  do  convex  lenses  form  inverted  images  of 
objects?  10.  When  is  the  image  larger  than  the  object?  11. 
When  is  it  smaller  than  the  object?  12.  When  does  a  convex 
lens  form  an  erect  image  of  an  object  ?  13.  Why  ?  14.  What  is 


NATURAL  PHILOSOPHY.  143 

the  size  of  the  image  compared  with  that  of  the  object? 
15.  Why?  1 6.  What  effects  do  concave  lenses  produce  on 
rays  of  light  by  refraction?  17.  What  images  of  objects  do 
concave  lenses  form  ?  18.  What  is  the  size  of  these  images 
compared  with  that  of  the  objects  ?  19.  Why  ?  20.  Describe 
a  camera  obscura.  21.  Describe  the  photographer's  camera. 
22.  Describe  the  eye.  23.  What  kind  of  an  optical  instrument 
is  the  eye. 


VII. 

ELECTRICITY. 


CHAPTER    XXII. 

FRICTIONAL   ELECTRICITY. 

144.  Electrification    by    Friction.  —  Many   sub- 
stances  when    rubbed   together   become    electrified, 

or  charged  with 
electricity  ;  that  is, 
they  acquire  the 
power  of  attracting 
and  repelling  light 
bodies. 

Illustration.  —  Rub 
a  dry  glass   rod  with 
silk,  and  hold  it  near 
bits  of  paper  or  pieces 
Fig-  I3°-  of    pith,    and    it    will 

attract  these  bodies ;  and,  if  it  is  strongly  excited,  it  will  after- 
wards repel  them  (Figure  130).  A  piece  of  sealing-wax  or  of 
vulcanite,  when  rubbed  with  flannel  or  fur,  also  acquires  the 
same  property  of  attracting  light  bodies. 

145.  Conductors    and   Insulators.  —  Metals    and 
certain    other   substances    allow  electricity  to   pass 

"44 


NATURAL  PHILOSOPHY.  145 

off  readily  through  them.  Such  substances  are 
called  conductors.  Other  substances,  such  as  glass, 
silk,  sealing-wax,'  and  dry  air,  will  not  allow  elec- 
tricity to  pass  through  them.  Such  substances  are 
called  insulators.  A  conductor  when  entirely  sur- 
rounded by  insulators  is  said  to  be  insulated.  A 
brass  cylinder  supported  on  a  glass  rod,  or  a  pith 
ball  hung  on  a  silk  thread,  is  insulated. 

Illustrations.  —  A  brass  rod  held  in  the  hand,  and  rubbed 
with  a  cat-skin,  does  not  become  electrified;  the  electricity 
passing  off  through  the  hand  as  rapidly  as  it  is  developed. 
A  brass  cylinder  when  supported  on  a  glass  rod  becomes 
powerfully  electrified  when  stroked  with  the  cat-skin ;  the  glass 
rod  preventing  the  escape  of  the  electricity. 

146.  Two  Kinds  of  Electricity.  —  The  electricity 
which  is  excited  on  glass  when  rubbed  with  silk 
is  the  opposite  in  kind  to  that  excited  on  vulcanite 
when  rubbed  with  fur.  The  former  kind  of  elec- 
tricity is  called  positive  electricity ;  and  the  latter, 
negative  electricity.  Bodies  charged  with  unlike 
electricities  attract,  and  those  charged  with  like  elec- 
tricities repel,  each  other. 

Illustration.  —  Excite  a  glass  rod  by  rubbing  it  with  silk, 
and  present  it  to  a  pith  ball  hung  on  a  silk  thread  (Figure 
131).  The  ball  will  at  first  be  attracted  to  the  rod,  and  then 
repelled  from  it.  After  it  has  once  been  repelled,  it  can  no 
longer  be  made  to  touch  the  glass  rod.  Excite  now  a  vul- 
canite tube  by  rubbing  it  with  fur,  and  present  it  to  a  second 
pith  ball  hung  on  a  silk  thread.  This  pith  ball  will  also  be 
attracted,  and  then  repelled.  Present  the  excited  vulcanite  to 
the  ball  which  is  repelled  by  the  glass,  and  it  will  be  attracted. 
Present  the  excited  glass  to  the  ball  that  is  repelled  by  the 
vulcanite,  and  it  will  also  be  attracted.  Bring  the  two  pith 


146 


NATURAL  PHILOSOPHY. 


balls  slowly  together,  and  they  will  be  found  to  attract  each 
other.  Charge  the  two  pith  balls  by  allowing  both  to  come 
in  contact  with  either  the  vulcanite  or  the  glass,  and  they  will 
repel  each  other. 

147.  Electrical  Machines.  —  A  machine  for  de- 
veloping electricity  by  friction  consists  of  a  glass 
plate  arranged  so  as  to  turn  between  two  rubbers. 
One  form  of  the  machine  is  shown  in  Figure  132, 


Fig.  131. 

The  brass  cylinders,  which  are  supported  upon  glass 
rods,  form  what  is  called  the  prime  conductor.  In 
small  machines  a  single  cylinder  is  used.  Negative 
electricity  is  developed  on  the  rubbers,  and  conveyed 
to  the  earth.  Positive  electricity  is  developed  on 
the  glass,  and  passes  to  the  prime  conductor. 

148.  Electrical  Induction.  —  If  an  insulated  con- 
ductor is  placed  near  the  prime  conductor  of  an 
electrical  machine  in  action  (Figure  133),  the  prime 


NATURAL  PHILOSOPHY.  H7 

conductor  will  act  upon  it  by  induction  through  the 
air  which  separates  the  two.  No  electricity  passes 
from  the  prime  conductor  to  the  other;  but  nega- 
tive electricity  is  developed  on  the  near  end  of  the 
insulated  conductor,  and  positive  electricity  on  its 
farther  end.  A  charged  body  always  acts  by  indue- 


Fig.  132. 

tion  on  all  surrounding  conductors ;  driving  its  own 
kind  of  electricity  to  the  farther  end  of  these  con- 
ductors, and  drawing  the  opposite  electricity  to  the 
near  end. 

If  the  conductor  is  removed  from  the  machine 
without  touching  the  cylinder,  it  loses  all  trace  of 
electricity.  If  we  touch  the  cylinder  with  the 
finger  before  we  remove  the  conductor,  we  shall 


148 


NATURAL   PHILOSOPHY. 


find,  on  removing  the  conductor,  that  it  is  charged 
with  negative  electricity.  The  positive  electricity 
was  driven  off  through  the  hand  into  the  earth. 


Fig-  133- 

The  finger  must  be  removed  from  the  conductor 
before  the  conductor  is  taken  from  the  influence 
of  the  machine. 


Fig.  134- 

Illustration.  —  Balance  an  ordinary  lath  on  a  piece  of  glass 
or  vulcanite,  as   shown   in   Figure    134.  and  place   some  light 


NATURAL  PHILOSOPH^. 


149 


bodies  on  a  stand  under  one  end  of  it,  and  then  bring  an 
excited  vulcanite  tube  near  the  other  end  :  the  light  bodies 
will  immediately  be  attracted. 

149.  The  Electrophorus. — The  electrophorus  con- 
sists of  a  cake  of  wax  provided  with  a  metallic  lid 
which  has  an   insulating  handle.     The  wax   is  first 
excitedly  stroking  it  with  cat-skin.     The  lid  is  then 
placed    upon    it,    and    touched   with    the   finger   as 
shown  in  Figure  135.'    On   removing  the  finger,  and 
raising  the   disk,   a   spark 

may  be  drawn  from  it, 
as  shown  in  Figure  136. 
This  operation  may  be 
repeated  any  number  of 
times  without  re-exciting, 
the  wax.  The  disk  be- 
comes charged  by  induc- 
tion. No  electricity  passes 
from  the  wax  to  the  lid. 

150.  The   Holtz    Elec- 
trical   Machine.  —  In  the 
Holtz   machine  electricity 

is  developed  by  induction.  This  machine  is  shown 
in  Figure  137.  It  consists  essentially  of  two  plates 
of  glass  near  together,  one  of  which  is  stationary, 
and  the  other  capable  of  rotating.  There  are  two 
pieces  of  paper  pasted  upon  the  stationary  plate, 
at  opposite  parts.  One  of  these  pieces  of  paper 
is  excited  by  friction,  and  acts  like  the  excited  wax 
of  the  electrophorus.  As  the  rotating  plate  turns 
in  front  of  this  paper,  electricity  is  developed  on 
it  by  induction,  and  passes  off  to  the  two  rods 


150 


NATURAL   PHILOSOPHY. 


at   the  front.      When   this   machine   is   in   action,   a 
perfect   torrent   of  sparks  will  pass   between   these 

two  rods  when  sep- 
arated from  each 
other. 

151.  Spark  Dis- 
charge. —  The  pas- 
sage of  electricity 
from  one  body  to 
another  is  called 
electric  discharge. 
The  most  common 
form  of  electric  dis- 
charge is  the  spark 
disc/large.  This  is 
the  usual  form  of 
discharge  in  the  air 
of  ordinary  density. 
The  light  of  the 

spark  is  due  to  the  fact  that  the  air  is  heated 
white-hot  by 
the  passage  of 
the  electricity 
through  it. 
The  noise  of 
the  spark  is 
due  to  the 
sudden  expan- 
sion and  con- 
traction of  the 
air,  as  it  is  suddenly  heated  by  the  passage  of  the 
electricity,  and  as  it  quickly  cools  after  the  passage. 


NATURAL  PHILOSOPHY. 


151 


Fig.  138. 


152  NATURAL   PHILOSOPHY. 

Illustrations,  —  When    the    knuckle    or  other  conductor   is 


Fig.  139. 

presented  to  a  charged  body,  a  spark  passes  between  the  two. 
When  the  rods  of   a  powerful  Holtz  machine  are  separated 


Fig.  140. 

twelve  or  fifteen  inches,  brilliant  zigzag  sparks  pass  between 


NATURAL  PHILOSOPHY.  153 

them,  as   shown   in    Figure  138.     Lightning  is  simply  a  spark 


discharge,  on  an  enormous  scale,  between  two  clouds,  or  be- 


Fig.  142. 

tween  a  cloud  and  the  earth  (Figure  139).     Thunder  is  merely 
the  intensified  sound  of  the  spark. 


154 


NATURAL  PHILOSOPHY. 


152.  Silent  Discharge.  —  It  is  found  impossible 
to  charge  to  any  extent  the  prime  conductor  of 
an  electrical  machine  or  other  body  which  has  a 
metallic  point  attached  to  it,  or  held  near  it,  as 


Fig.  143- 

shown   in    Figure    140.     The  point   causes  a   silent 
discharge. 

Illustration.  —  A  building  maybe  protected  from  injury  by 
lightning  by  placing  upon  it  a  metallic  rod  running  well  into 
the  ground  at  the  bottom,  and  well  pointed  at  the  top,  as 


shown  in  Figure  141.  The  point  serves  to  carry  off  the  elec- 
tricity silently  from  the  cloud.  The  rod  also  serves  as  a  path 
for  the  electricity  to  escape  to  the  earth  in  case  a  violent 
discharge  takes  place- 


NATURAL  PHILOSOPHY.  155 

153.  Auroral  Discharge.  —  When  the  discharge 
takes  place  through  a  vessel  containing  highly  rare- 
fied air,  we  get  a  soft  diffused  band  of  light,  as 
shown  in  Figure  142,  instead  of  the  sharp  line  of 
the  spark  discharge.  This  discharge  is  also  silent. 
It  is  called  the  auroral  discharge. 

Illustrations.  —  The  Northern  Lights  are  caused  by  an  auro- 
ral discharge  high  up  in  the  atmosphere.  This  light  usually 
appears  in  the  form  of  one  or  more  arches,  as  shown  in 
Figure  143,  which  sometimes  resemble  an  immense  curtain 
hanging  in  folds,  as  shown  in  Figure  144.  These  arches  are 
usually  surmounted  by  a  number  of  streamers. 

QUESTIONS. 

i.  How  may  bodies  be  electrified  ?  2.  Give  some  examples. 
3.  What  do  we  mean  by  conductors  ?  4,  By  insulators  ?  5.  By 
an  insulated  conductor?  6.  Name  some  conductors.  7.  Some 
insulators.  8.  How  many  kinds  of  electricity  are  there? 
9.  Name  them.  10.  How  may  we  develop  each?  11.  When 
do  electrified  bodies  attract,  and  when  repel,  each  other? 
12.  Give  illustrations.  13.  Describe  the  ordinary  electrical 
machine.  14.  What  is  meant  by  electrical  induction?  15. 
What  takes  place  in  induction?  16.  What  is  the  state  of  a 
conductor  after  it  has  been  removed  from  a  charged  body 
without  having  been  touched  by  the  finger?  17.  When  it  has 
been  removed  after  having  been  touched  with  the  finger? 
1 8.  Describe  the  electrophorus.  19.  Describe  its  use.  20.  By 
what  means  is  the  lid  charged?  21.  What  electrical  machine 
develops  electricity  by  induction  ?  22.  DescVibe  this  machine. 
23.  What  is  meant  by  electric  discharge?  24.  What  is  the 
most  common  form  of  electric  discharge  ?  25.  To  what  is 
the  light  of  the  spark  due  ?  26.  The  sound  of  the  spark  ? 
27.  Give  illustrations  of  the  spark  discharge.  28.  What  is  light- 
ning? 29.  What  is  the  cause  of  thunder?  30.  What  is  the 
effect  of  points  on  charged  bodies?  31.  For  what  purpose  is 


156 


NATURAL   PHILOSOPHY. 


a  lightning-rod  used  ?  32.  Describe  the  rod,  and  tell  in  whal 
two  ways  it  acts.  33.  What  is  meant  by  the  auroral  discharge  ? 
34.  What  are  the  Northern  Lights  ?  35.  Of  what  two  por- 
tions are  the  Northern  Lights  usually  composed? 


CHAPTER    XXIII. 

VOLTAIC    ELECTRICITY. 

154.  Voltaic  Cell.  --The  simplest  form  of  a 
voltaic  cell  consists  of  a  plate  of  zinc  and  a  plate 
of  copper  immersed  in  dilute  sulphuric  acid,  as 
chown  in  Figure  145.  The  ends  of  the  plates  which 
are  above  the  liquid  are  connected  by  means  of  ? 
copper  wire.  When  the  plates  are  thus  connected, 
there  is  a  steady  flow  of 
electricity  through  the  wire 
from  the  copper  to  the  zinc, 
and  through  the  liquid  from 
the  zinc  to  the  copper.  This 
flow  of  electricity  is  called 
the  electric  current.  The 
electricity  developed  in  this 
way  by  the  action  of  the 
liquid  upon  the  plate  of  a 
cell  is  called  voltaic  electricity,  from  Volta,  an 
Italian  philosopher,  who  was  one  of  the  first  to 
study  it. 

There  is  a  great  variety  of  voltaic  cells.  A  cell 
in  common  use,  and  called  Bunseri  s  cell,  is  shown 
in  Figure  146.  It  consists  of  a  vessel  A,  which  is 
filled  with  dilute  sulphuric  acid.  In  this  is  placed 


Fig-  T4s- 


NATURAL   PHILOSOPHY. 


157 


a  cylinder  of   zinc  B,   and   within   this   cylinder  of 
zinc    is    placed    the    porous   cup    Ct  which    contains 


nitric  acid ;  and  finally  the  rod  of  carbon  D  is 
placed  within  the  porous  cup.  DanieWs  cell  is 
shown  in  Figure  147.  The  outer  vessel  A  con- 
tains a  cylin- 
der of  copper 
and  a  solution 
of  blue  vitriol. 
The  inner  cup 
P  contains  the 
zinc  plate  and 
dilute  sulphuric 
acid. 

A       number 
of  voltaic  cells 

connected      to-  Fig^y.  •-, 

gether   constitute   what   is   called   a   voltaic    battery. 
The  free  zinc   plate   at   one   end   of   the   battery  is 


NATURAL  PHILOSOPHY. 


its  negative  pole,  and  the  free  carbon  or  copper  at 
the  other  end  is  the  positive  pole.  The  wire  which 
connects  the  two  poles  is  called  the  circuit. 

155.  The  Voltaic  Arc.  —  When,  a  powerful  cur- 
rent of  electricity  is  sent  through  two  carbon  points 
a  and  b  (Figure  148),  which  have  first  been  brought 
into  contact,  and  then  separated  a  little,  a  very  bril- 


liant luminous  arc  is  obtained.  This  arc  is  called 
the  voltaic  arc.  The  light  and  heat  of  this  arc  are 
the  most  intense  that  can  be  obtained  by  artificial 
means.  The  voltaic  arc  is  now  extensively  used  for 
outdoor  illumination,  as  shown  in  Figure  149.  The 
electric  currents  employed  for  this  purpose  are  not 
usually  developed  by  a  battery,  but  by  another 
method,  which  will  be  described  hereafter, 


NATURAL   PHILOSOPHY 


59 


156.  Illumination  by  Incandescence.  —  When  a 
powerful  currerrt  of  electricity  is  sent  through  a 
fin?  wire  or  rod  of  any  substance,  the  wire  or  rod 
is  heated  white-hot,  and  glows  with  a  brilliant  light. 


Such  wires  or  rods  are  used  for  illumination  by  in- 
candescence. The  chief  difficulty  in  this  method  is, 
that  a  wire  made  of  any  known  substance  is  very 
liable  to  melt  off  when  a  powerful  current  is  sent 
through  it ;  and  a  fine  rod  of  any  substance  like 


i6o 


NATURAL  PHILOSOPHY. 


carbon,  which  will  not  melt,  is  liable  to  be  burned 
up  in  the  intense  heat.  The  method  of  incandes- 
cence is  preferable  for  indoor  illumination. 

Figure  150  shows  one  form  of  lamp  employed  for 
this  kind  of  illumination.  The  upper  portion  of 
the  lamp  is  a  glass  globe  from  which  the  air  has 
been  exhausted,  and  which  is  sealed  air-tight.  In 
the  centre  of  this  globe  is  a  car- 
bon filament,  bent  in  the  form  of 
a  ring.  The  ends  of  this  filament 
are  held  in  little  clamps  connected 
with  platinum  wires,  which  pass 
through  the  glass  of  the  smaller 
globe  under  the  ring,  and  thence 
out  through  the  bottom  of  the 
lamp,  where  they  are  connected 
with  the  wires  of  the  circuit. 

157.  Electrolysis. — When  a  cur- 
rent of  electricity  is  sent  through 
water  or  other  compound  liquid, 
as  shown  in  Figure  151,  the  liquid 
is  decomposed.  The  current  is  in- 
troduced into  the  liquid  by  means 
of  metallic  strips  seen  at  the  bottom  of  the  vessel. 
Water  is  decomposed  into  hydrogen  and  oxygen  ; 
and  these  gases  may  be  collected  in  tubes,  as  shown 
in  the  figure.  The  decomposition  of  a  substance 
by  means  of  the  electric  current  is  called  electroly- 
sis. The  two  metallic  strips  by  which  the  current 
is  introduced  into  the  liquid  are  called  the  elec- 
trodes :  the  one  connected  with  the  positive  pole  of 
the  battery,  the  anode ;  and  the  one  connected  with 


Fig.  150. 


NATURAL  PHILOSOPHY. 


161 


the  negative  pole,  the  cathode.  When  the  liquid 
contains  a  compound  of  a  metal,  the  metal  is  always 
set  free  at  the  cathode,  and  is  deposited  upon  it. 

This  deposition  of  metals  by  means  of  the  electric 
current  is  called  electro-metallurgy,  and  is  of  great 
practical  importance.  The  two  chief  processes  of 
electro-metallurgy  are  electrotyping  and  electroplat- 
ing. The  former  is  copying  by  means  of  electricity, 
and  the  latter  is  coating  the  baser  metals  ^vith  the 
more  noble  by  means  of  electricity. 


Fig.  IS1- 

158.  Electrotyping. — Any  thing  may  be  elec- 
trotyped  of  which  a  mould  may  be  taken  in  wax. 
The  chief  use  of  electrotyping  is  in  copying  the 
face  of  printers'  type  arid  wood- engravings,  after 
they  have  been  arranged  for  the  pages  of  a  book. 

A  mould  is  first  taken  in  wax  of  the  article  to 
be  copied,  and  the  wax  is  coated  with  a  thin  film 
of  some  conducting  substance,  such  as  graphite 
powder.  The  mould  is  then  hung  up  in  a  trough 
filled  with  a  solution  of  sulphate  of  copper  (blue 


1 62 


NATURAL  PHILOSOPHY. 


vitriol),  called  the  bath.  The  mould  is  connected 
with  the  negative  pole  of  the  battery  (Figure  152), 
so  as  to  make  it  a  cathode.  A  plate  of  copper  is 
hung  in  the  bath  opposite  the  mould,  and  connected 
with  the  positive  pole  of  the  battery,  so  as  to  make 
it  an  anode.  On  the  passage  of  the  current  through 
the  bath,  copper  is  deposited  from  the  solution  upon 
the  mould  in  a  uniform  sheet.  The  moulds  are 


Fig.  152. 

usually  hung  in  the  bath  at  night,  and  in  the  morn- 
ing they  are  removed,  and  the  wax  melted  off.  The 
copper  casts  are  made  sufficiently  firm  for  use  in 
printing  by  backing  them  with  type-metal. 

159.  Electroplating. — Table-ware,  such  as  knives, 
forks,  tea-sets,  etc.,  is  plated  with  silver  by  elec- 
trolysis. The  article  to  be  plated  is  very  carefully 
cleaned,  and  hung  in  a  bath  containing  a  solution  of 
cyanide  of  silver.  It  is  then  connected  with  the 
negative  pole  of  a  battery  (Figure  153),  while  a  piece 


NATURAL  PHILOSOPHY.  163 

of  silver  opposite  it  is  connected  with  the  positive 
pole.      On  the  passage  of  the  current,  silver  is  de- 


Fig.  153. 

posited  from  the  solution   upon  the  article  forrning 
the  cathode,  to  which  it  firmly  adheres. 

QUESTIONS. 

i.  Describe  a  simple  voltaic  cell.  2.  Bunsen's  cell.  3. 
Darnell's  cell.  4.  What  is  a  voltaic  battery  ?  5.  What  are  its 
poles?  6.  What  is  the  circuit?  7.  What  is  the  voltaic  arc? 
and  how  is  it  obtained?  8.  What  is  true  of  its  light  and  heat? 
9.  What  is  now  a  common  use  of  this  arc?  10.  What  is  illu- 
mination by  incandescence  ?  1 1.  What  are  the  chief  difficulties 
in  this  method?  12.  Describe  one  form  of  lamp  used.  13. 
What  is  electrolysis?  14.  Give  an  illustration.  15. 'What  is 
meant  by  the  electrodes  ?  16.  By  the  anode  ?  17.  By  the  cath- 
ode ?  18.  What  takes  place  when  the  liquid  contains  a  me- 
tallic compound  in  solution?  19.  What  is  electro-metallurgy? 
20.  What  are  its  two  chief  processes?  21.  What  is  electro- 
typing?  22.  What  is  its  chief  use?  23.  Describe  the  process 
24.  What  is  electroplating?  25.  Describe  the  process. 


VIIL 

ELECTRO-MAGNETISM. 


CHAPTER    XXIV. 

ELECTRO-MAGNETS. 

160.   Permanent  Steel  Magnets.  —  Magnets    are 
recognized  by  their  peculiar  property  of  attracting 

iron.      Ordinary  mag- 


nets are  bars  of  steel, 
either  straight,  or 
bent  into  the  form  of 
a  horseshoe.  A  mag- 
net poised  so  as  to 
turn  freely,  as  shown 
in  Figure  154,  always 
takes  a  north  and 
south  direction.  Such 
a  magnet  is  called  a 
magnetic  needle.  The 
end  of  the  magnet 
which  points  towards 
the  north  is  called 


Fig.  154. 


its  north  pole ;   and   the   end   which   points  towards 
the  south,  the  south  pole.     The   power  of   a  mag- 
164 


NATURAL   PHILOSOPHY. 


I65 


net    resides    chiefly  in    its    poles.      Unlike  poles   of 
magnets   attract,,  and  like  poles  repel,  each  other. 


Fig.  156. 


Fig.  155. 

Illustrations.  —  Place  a  bar  magnet  in  iron  filings:  on  re- 
moving the  bar,  the  filings  will  be  seen  to  cling  chiefly  to  the 
ends  of  the  magnet.  Present  the  north  pole  of  a  bar  magnet 
to  the  north  pole  of  a  magnetic  needle.  The  needle  will  be 
repelled.  Present  the  south  pole  of  the  bar  to  the  north  pole 
of  the  needle.  The  needle  will  be  attracted. 

161.  Magnetic  Induction.  —  When  a 
piece  of  iron  is  brought  near  a  magnet, 
or  in  contact  with  it,  the  iron  becomes 
magnetic  by  induction.  As  soon  as  the 
iron  is  removed  from  the  influence  of 
the  magnet,  it  loses  its  magnetism.  The 
iron  and  the  magnet  act  upon  each 
other  in  such  a  way  that  any  movement 
of  the  iron  near  the  magnet  changes  the 
strength  of  the  magnet.  When  a  piece 
of  steel  is  brought  in  contact  with  a 
magnet,  it  also  becomes  magnetic  by 
induction  ;  but,  when  it  is  removed  from 
the  magnet,  it  retains  its  magnetism. 

Illustrations.  —  Place  a  small  nail  upon  the 
pole  of  a  large  bar  magnet.  The  nail  will 
become  magnetic,  and  will  be  able  to  take  up  a 
second  nail,  which,  in  turn,  will  become  mag- 
netic, and  take  up  a  third ;  and  so  on.  Rub 
a  knife-blade  upon  a  magnet,  and  it  will  acquire  the  power  of 
taking  up  a  tack,  and  will  retain  this  power  for  a  long  time. 


1 66  NATURAL  PHILOSOPHY. 

162.  Galvanometers.  —  When    a   wire    through 
which  an  electric  current  is  passing  is  held  over  a 
magnetic  needle,  as  shown  in  Figure  155,  the  needle 
is  turned  aside,  or  deflected.     If   the  same  wire  is 
held  under  the  needle,  it  is  deflected  in  the  oppo- 
site direction.     The  needle  seeks  to  place  itself  at 
right  angles  to    the   wire.     When    the  wire   passes 
around    the  needle,    as    shown    in    Figure    156,    its 
action    on    the    needle    is    more   powerful;    and    it 
may  be  still  further  increased  by  winding  the  wire 

several  times  around  the 
needle.  A  needle  surround- 
ed by  a  coil  of  wire  is 
called  a  galvanometer.  It 
serves  to  show  the  presence, 
the  strength,  and  the  direc- 
tion of  the  current  in  the 
'wire  to  which  it  is  attached. 
It  shows  the  first  by  the 
deflection  of  the  needle,  the 
second  by  the  amount  of 

its    deflection,    and    the    third    by    the    direction'  in 

which  it  is  deflected. 

163.  Electro-Magnets.  —  When  a  current  of  elec- 
tricity is  sent  through  a  wire  which  is  wound  around 
a  rod  of  soft  iron,  as  shown  in  Figure  157,  the  iron 
becomes  a  magnet.     The  iron  remains  a  magnet  as 
long  as  the  current  is  passing,  but  loses   its  mag- 
netism  the  instant  the  current  is  stopped.     A  piece 
of  iron  placed  thus  within  a  coil   of  wire  is  called 
an    electro-magnet.      These    magnets    are    far    more 
powerful  than  ordinary  steel  magnets  ;  and  they  can 


NATURAL  PHILOSOPHY.  1 67 

be  made  active  or  inactive  by  simply  starting  and 

stopping  the  current  in  the  wire.     Any  change  in 

the   current    in    the   coils,  whether   in    strength    or 

direction,  produces   a  change  of 

magnetism  in   the   iron.     These 

magnets  are  sometimes  straight, 

but   usually  bent   into  the  form 

of  a  letter  U,  as  shown  in  Fig-  Fis-  J59- 

ure   158.     When  bent   in  this  way,  the  wire  is  not 

coiled  around  the  whole  length  of  the  iron  bar,  but 

only  around  its  ends,  or  poles.     The  bent  portion  of 


Fig.  1 60. 

the  bar  is  often  replaced  by  a  straight  bar  of  iron, 
which  connects  the  rods  within  the  two  coils,  as 
shown  in  Figure  159.- 

164.  The   Morse    Key. — The   Morse  key   is   an 
instrument  for  opening  and  clos-  ^ 

ing  the  circuit,  so  as  to  stop  and  i  ^  rj&  **'' 
start  the  current  from  the  battery.  rj  c 

The  complete   key  is    shown    in  Fig-  161. 

Figure  160,  and  the  essential  parts  of  it  are  shown 
in  outline  in  Figure  161.  K  is  a  metallic  bar  called 
the  lever ;  a  is  the  axis  on  which  it  turns  ;  b  is  a 


1 68 


NATURAL   PHILOSOPHY. 


platinum  point  connected  with  the  lever  ;  c  is  a  sta- 
tionary platinum  point  directly  under  b,  called  the 
anvil  ;  and  d  is  a  vulcanite  button  by  which  the 
lever  is  pressed  down.  There  is  a  spring  under 

the  lever  of  the  key 
which  keeps  it  up  so 
as  to  separate  the 
platinum  points  when 


B 

Fig.  162. 


the       lever      is     not 
pressed  down. 

In  Figure  162  the 
key  is  shown  in  the  circuit  of  a  battery.  One  pole 
of  the  battery  is  connected  with  the  anvil  by  a 
wire,  and  the  other  with  the  lever  at  the  axis. 


Fig.  163. 

When  the  lever  is  up,  the  circuit  is  opened  at  a 
by  the  separation  of  the  platinum  points,  and  the 
current  is  stopped.  When  the  lever  is  pressed 
down,  the  circuit  is  closed  by  the  contact  of  the 
platinum  points  at  a,  and  the  current  starts. 


NATURAL  PHILOSOPHY.  169 

165.  The  Telegraphic  Sounder.  —  The  sounder 
is  shown  in  Figure  163,  and  its  essential  parts  in 
outline  in  Figure  164.  A  is  an  electro-magnet ;  L 
is  a  lever ;  b  is  the  axis  on  which  the  lever  turns ; 
c  is  a  spring  which  pulls  the  lever  up  ;  .e  is  a  piece 
of  soft  iron,  fastened  across  the  lever  just  over  the 
electro-magnet ;  and  d  is  a  piece  of  metal  against 
which  the  lever  strikes  when 

*  C  T 

it  is  drawn  down. 

Figure  165  shows  the  sound- 
er and  key  in  circuit.  One 
pole  of  the  battery  is  con- 
nected by  a  wire  with  the 
circuit  of  the  key ;  the  other  pole  is  connected  with 
one  end  of  the  wire  of  the  electro-magnet  of  the 
sounder ;  and  the  other  end  of  this  wire  magnet  is 
connected  with  the  lever  of  the  key  at  the  axis. 

When    the    lever   of   the   key  is   up,   the    circuit 
g  is    broken    at    a, 

^ mm   fi  K  ^.    tne     current     is 

stopped,  the  elec- 


i* J         tro-magnet  of  the 

B   '  sounder   is    inac- 

Fis-  l65-  tive,  and  the  lever 

of  the  sounder  is  thrown  up  by  the  spring.  If  the 
lever  of  the  key  is  pushed  down,  contact  is  made 
at  a,  which  closes  the  circuit ;  the  current  starts, 
the  electro-magnet  of  the  sounder  becomes  active, 
and  the  lever  of  the  sounder  is  drawn  down  by  the 
pull  of  the  magnet  upon  the  iron  above  it.  As 
the  lever  is  drawn  down,  it  clicks  from  striking  the 
metallic  stop  at  the  end. 


I/O  NATURAL  PHILOSOPHY. 

The  clicking  of  the  sounder  is  controlled  by  the 
key,  even  when  these  are  miles  apart ;  for  the 
sounder  clicks  every  time  the  lever  of  the  key  is 
depressed.  Letters  and  words  are  indicated  by  com- 
binations of  long  and  short  intervals  between  the 


Fig.  166. 

clicks.  The  operator  listens  to  the  sounder  just  as 
we  listen  to  a  person  who  is  talking  to  us,  and 
soon  becomes  able  to  follow  it  as  readily. 

166.  The  Telegraphic  Relay.  —  On  long  lines, 
in  which  there  are  a  number  of  stations,  the  current 
from  the  main  battery  is  not  strong 
enough  to  work  the  sounders  with 
sufficient  force.  This  necessitates 
the  use  of  an  instrument  called 
the  relay  (Figure  166),  by  means 
of  which  a  local  battery  is  made 
to  work  the  sounder.  Its  essential  parts  are  shown 
in  outline  in  Figure  167.  A  is  an  electro-magnet ; 
/  is  the  lever,  which  turns  upon  an  axis  at  b ;  c  is 
a  piece  of  soft  iron  fastened  across  the  lever  in 
front  of  the  electro-magnet ;  f  is  a  spring  for  pull- 


NATURAL  PHILOSOPHY.  171 

ing  the  lever  back ;  d  and  e  are  two  platinum 
points,  the  former  fastened  to  the  lever,  and  the 
latter  stationary. 

Figure  168  shows  the  way  in  which  the  key, 
relay,  and  sounder  are  connected.  The  full  line 
represents  the  circuit  of  the  main  battery  M ;  and 
the  dotted  line,  of  the  local  battery  L.  One  pole 
of  the  main  battery  is  connected  with  the  anvil  of 
the  key,  and  the  other  with  one  end  of  the  wire  of 
the  electro-magnet  of  the  relay.  The  other  end 
of  the  wire  of  this  magnet  is  connected  with  the 
lever  of  the  key  at  the  axis.  One  pole  of  the 
local  battery  is  connected  to  the  lever  of  the  relay, 
and  the  other  pole  to  the  electro-magnet  of  the 
sounder,  and  then  to  the  stationary  platinum  point 
of  the  relay.  When  the  lever  of  the  key  is  up, 
the  main  circuit  is  opened  at  a;  the  current  is 
stopped,  the  electro-magnet  of  the  relay  is  inactive, 
the  lever  of  the  relay  is  drawn  back  by  the  spring, 
the  local  circuit  is  opened  at  b  by  the  separation 
of  the  platinum  points,  the  electro-magnet  of  the 
sounder  is  inactive,  and  the  bar  of  the  sounder  is 
thrown  up  by  the  spring.  When  the  lever  of  the 
key  is  pushed  down,  contact  is  made  at  a,  the  main 
circuit  is  closed,  the  electro-magnet  of  the  relay 
becomes  active,  the  lever  of  the  relay  is  drawn 
forward,  contact  is  made  at  b,  the  local  circuit  is 
closed,  the  electro-magnet  of  the  sounder  becomes 
active,  and  the  lever  of  the  sounder  is  drawn  down. 
Thus  the  levers  of  the  relay  and  sounder  vibrate  in 
unison,  but  each  is  worked  by  a  different  battery. 
The  vibration  of  the  lever  of  the  relay  is  controlled 


172 


NATURAL  PHILOSOPHY, 


L 1 


•DC 


Fig.  168. 


NATURAL  PHILOSOPHY.  1/3 

by  the  key,  and  controls  the  vibration  of  the  lever 
of  the  sounder  by  opening  and  closing  the  local 
circuit. 

QUESTIONS. 

i.  What  is  the  distinguishing  property  of  a  magnet?  2. 
What  are  the  two  forms  of  magnets  ?  3.  What  is  a  magnetic 
needle?  4.  What  are  the  north  and  south  poles  of  a  magnet? 
5.  How  do  like  and  unlike  poles  of  magnets  act  upon  each 
other?  6.  Give  illustrations.  7.  What  is  meant  by  magnetic 
induction  ?  8.  What  is  the  difference  in  the  effect  upon  iron 
and  steel  when  brought  under  the  influence  of  a  magnet?  9. 
What  effect  has  every  movement  of  a  piece  of  iron  near  a 
magnet  upon  its  magnetism?  10.  What  is  the  effect  of  a  cur- 
rent of  electricity  upon  a  magnetic  needle  ?  IT.  How  may  this 
effect  be  increased  ?  12.  What  is  a  galvanometer?  13.  What 
are  its  uses?  14.  What  is  an  electro-magnet?  15.  How  does 
it  compare  with  a  steel  magnet  in  strength  ?  16.  How  may  it 
be  rendered  active  and  inactive?  17.  For  what  is  the  Morse 
key  used?  18.  Describe  the  instrument.  19.  Describe  its  con- 
nection with  the  circuit  of  a  battery.  20.  Explain  how  it 
opens  and  closes  the  circuit.  21.  Describe  the  telegraphic 
sounder,  22.  Describe  how  it  and  the  key  may  be  connected 
in  the  same  circuit.  23.  Explain  how  the  key  may  make  the 
sounder  click.  24.  What  may  be  indicated  by  the  clicks  of 
the  sounder  ?  25.  In  what  way  ?  26.  Why  is  it  necessary  to 
use  the  relay?  27.  Describe  this  instrument.  28.  Describe 
the  way  in  which  the  relay,  sounder,  and  key  are  connected. 
29.  Explain  how  the  vibration  of  the  key  makes  the  levers  of 
the  relay  and  of  the  sounder  vibrate  in  unison.  30.  Why  do 
the  batteries  make  each  lever  vibrate?  31.  Why  do  the  levers 
vibrate  in  unison  ? 


174  NATURAL  PHILOSOPHY. 

CHAPTER    XXV. 

MAGNETO-ELECTRICITY. 

167.   Currents  induced   by  Magnetism.  —  When 

a  magnet  and  a  wire  whose  ends  are  connected 
are  moved  near  each  other,  a  current  of  electricity 
is  developed  by  induction  in  the  wire.  The  current 
continues  only  while  the  motion  continues  ;  and,  if 
the  motion  is  to  and  fro,  the  direction  of  the  current 
is  changed  every  time  the  direction  of  the  motion  is 
changed. 

When  the  magnet  and  wire   are  stationary,  any 
change  in  the  magnetism  of  the  magnet  will  induce 
a  current  in  the  wire,  which 
will  continue  only  while  the 
change  is  taking  place. 

When  two  wires  are  near 
each  other,  and  a  current  is 
flowing  through  one  of  them, 
any  movement  of  the  wires 
with  respect  to  each  other, 
-  l69-  or  any  change  in  the  cur- 

rent in  the  one  wire,  will  induce  a  current  in  the 
other  wire.  The  current  developed  in  any  of  the 
above  ways  is  called  an  induced  current,  or  a 
magneto-electric  current. 

Illustrations.  —  If  a  magnet  N  S  (Figure  169)  is  moved  sud- 
denly in  or  out  of  the  coil  of  wire,  a  current  will  be  induced 
in  the  coil,  which  will  be  in  one  direction  on  inserting  the  pole, 
and  in  the  other  on  withdrawing  it.  If  the  magnet  is  reversed, 
so  as  to  use  the  other  pole,  the  current  will  be  reversed. 


NATURAL   PHILOSOPHY. 


175 


If  the  magnet  is  placed  within  the  coil,  and  its  magnetism 
is  changed  by  moving  a  piece  of  iron  to  and  fro  near  the  pole, 
a  current  will  be  induced  in  the  coil. 

If  a  coil  of  wire  through  which  a  current  is  passing  is  used 
instead  of  a  steel  magnet  (Figure  170),  precisely  similar  results 
are  obtained.  The  more  suddenly  the  steel  magnet,  or  the  coil 
conveying  a  current,  is  moved  in  or  out  of  the  coil,  the  stronger 
the  current  induced. 

If  the  small  coil  is  left  within  the  larger  coil,  any  change, 
whatever  in  the  current  in  the  inner  coil,  whether  of  strength  or 
direction,  will  develop  a  current  by  induction  in  the  outer  coil. 

If  a  bar  of  soft  iron  is  inserted  in  the  inner  coil  of  Figure 
170,  the  current  induced  in  the  outer  coil,  either  by  motion  or 
change  of  current,  will  be  very  much  stronger. 


Fig.  170. 

168.  The   Induction   Coil. — The  induction   coil 
consists  of   two  coils  :    an  inner  or  primary  coil  of 
coarse  wire,  enclosing  pieces  of  soft  iron,  usually  in 
the  form  of  wires  *  and  an  outer  or  secondary  coil 
of  fine  wire.     The  coils  are  carefully  insulated  from 
each  other.     A  current  of  electricity  is  sent  through 
the  primary  coil  ;  and  any  change  in  the  strength  of 
this  primary  ctirrent  develops  by  induction  a  current 
in  the  secondary  coil. 

169.  The   Bell   Telephone. —  Figures    171     and 
172   show   the   construction   of   the  Bell   Telephone. 


NATURAL   PHILOSOPHY. 


It  consists  of  a  steel   magnet   M,  around  one  end 
of  which  is  wound  a  coil  of  fine  wire  B.     The  coil 


serves  as  a  handle. 


Fig.  171. 

and  magnet  are  enclosed  in  a  wooden  case,  which 
One  end  of  this  case  is  en- 
larged and  hollowed  out  at  E, 
so  as  to  serve  as  a  mouth- 
piece or  an  ear-piece.  A 
diaphragm  of  thin  iron  D  is 
stretched  across  the  wide  end 
of  the  case,  just  in  front  of 
the  pole  of  the  magnet,  which 
it  does  not  touch. 

The  transmitting  and  re- 
ceiving instruments,  which 
are  exactly  alike  in  construc- 
tion, are  connected  by  a  wire. 
If  a  person  speaks  into  the 
mouth-piece,  the  air  in  it  is 
thrown  into  vibration,  and 
the  vibrations  are  communi- 
cated to  the  diaphragm.  The 
Fig.  172-  vibrations  of  the  iron  plate 

produce  slight  temporary  alterations  in  the  magnet- 
ism of   the  steel  magnet.     These  changes  of   mag- 


NATURAL  PHILOSOPHY.  177 

netism  in  the  magnet  induce  corresponding  currents 
in  the  wire  of  the  coil,  which  are  transmitted  over 
the  wire  connecting  the  two  instruments :  hence 
pulsations  of  electricity  exactly  corresponding  to 
the  vibrations  of  the  diaphragm  of  the  first  instru- 
ment, will  be  transmitted  over  the  wire,  and  through 
the  coil  of  the  receiving  instrument.  These  pulsa- 
tions of  the  current  in  the  coil  will  induce  in  the 
magnet  of  the  receiving  instrument  exactly  the  same 
changes  of  magnetism  as  those  by  which  they  ibere  pro- 
duced in  the  sending  instrument.  These  changes  of 
magnetism  cause  the  magnet  to  pull  upon  the  iron 
plate  in  front  of  it  with  a  varying 
force,  and  consequently  to  make  it 
vibrate  exactly  like  the  diaphragm  of 
the  transmitter.  These  vibrations  are 
communicated  to  the  air,  and  then 
to  the  ear  of  the  operator,  which  is 
placed  at  the  mouth  of  the  receiver. 
The  words  spoken  into  the  transmitter  are  thus 
reproduced  in  the  receiver. 

170.  The  Carbon  Button.  —  The  carbon  button 
consists  of  a  disk  of  carbon  between  two  metallic 
plates,  which  are  placed  directly  against  it,  as  shown 
in  Figure  173.  Each  metallic  plate  is  connected 
with  one  of  the  poles  of  a  battery.  The  slightest 
variation  of  pressure  upon  these  plates  alters  the 
conducting  capacity  of  the  button,  and  changes  the 
strength  of  the  current  flowing  through  it.  An  in- 
crease of  pressure  makes  the  current  stronger,  and  a 
lessening  of  pressure  makes  it  weaker.  This  button 
is  exceedingly  sensitive  to  changes  of  pressure. 


NATURAL  PHILOSOPHY. 

171.  The  Edison  Telephone.  —  There  is  no  bat- 
tery used  in  the  Bell  telephone  ;  but  in  the  Edison 
telephone  a  battery  is  used,  and  the  current  from  the 
battery  is  thrown  into  pulsations  by  means  of  a 
carbon  button. 

One  form  of  the  Edison  transmitter  is  shown  in 
Figure  174.  The  mouth-piece  is  of  vulcanite.  Back 
of  this  is  the  vibrating  disk,  and  behind  this  is  a 
little  round  button  of  aluminium,  which  rests  upon 
the  metallic  plate  in  front  of  the  carbon  disk.  This 


Fig.  174. 

plate  is  of  platinum.  Behind  the  carbon  disk  is  a 
second  platinum  plate,  held  in  position  by  a  screw 
at  the  back  of  the  instrument.  The  battery  wires 
are  connected  with  the  two  platinum  plates  in  such  a 
way  that  the  current  must  traverse  the  carbon  disk. 
On  speaking  into  the  mouth-piece,  the  disk  is 
thrown  into  vibration.  The  vibrations  are  commu- 
nicated to  the  platinum  plate  and  the  carbon  disk 
by  means  of  the  aluminium  button,  thus  producing 
undulations  in  the  current  exactly  corresponding  to 
the  vibrations  of  the  disk. 


NATURAL  PHILOSOPHY.  179 

The  receiving  instrument  of  the  Edison  telephone 
is  similar  to  that  of  the  Bell  telephone.  Changes  of 
magnetism  are  induced  in  it  by  the  undulating  cur- 
rent which  traverses  its  coil ;  and  these  changes  of 
magnetism  cause  the  disk  in  front  of  the  magnet  to 
vibrate  exactly  like  that  of  the  transmitter. 

172.  The  Dynamo-Electric  Machine.  —  Power- 
ful currents  of  electricity,  such  as  those  used  for 
the  electric  light  and  for  a  variety  of  other  pur- 
poses, are  now  developed  by  magneto-electric  induc- 
tion. The  machines  used  are  called  dynamo-electric 
machines,  or  simply  dynamo  machines.  In  all  of 
these  machines  the  currents  are  developed  by  the 


Fig.  175.  Fig.  176. 

rotation  of  coils  of  wires  arranged  on  cylinders 
called  armatures,  between  the  poles  of  powerful 
magnets.  The  armature  of  the  Edison  machine  is 
shown  in  Figure  175,  and  a  section  of  it  is  shown 
in  Figure  176.  It  is  a  cylinder  of  wood,  through 
the  centre  of  which  passes  an  iron  rod,  upon  which 
it  rotates.  The  wood  is  first  wound  transversely, 
like  thread  on  a  spool,  with  iron  wire.  It  is  then 
wound  lengthwise  with  copper  wire.  The  whole 
machine  is  shown  in  Figure  177.  It  consists  of  a 
large  upright  electro-magnet,  the  poles  of  which  are 
the  large  pieces  of  iron  seen  at  the  bottom  of  the 
coils.  These  poles  are  hollowed  out  so  as  to  receive 


i8o 


NATURAL   PHILOSOPHY. 


the  armature,  which  they  nearly  enclose.     The  arma- 
ture is  rotated  by  means  of  the  pulley  and  belt  seen 

at  the  back.  As  the  cop- 
per wires  of  the  arma- 
ture are  carried  around 
past  the  poles  of  the 
magnet,  currents  are 
developed  by  induction. 
The  iron  wire  upon 
which  the  copper  is 
wound  increases  the 
strength  of  the  induced 
currents. 

It  has  been  found, 
that,  if  any  current  from 
an  outside  source  is 
sent  through  the  arma- 
ture of  any  of  these 
machines,  it  will  make 
the  armature  revolve  in 
the  opposite  direction  to 
that  in  which  it  would 
have  to  revolve  in  order 
to  develop  a  similar  current :  hence,  by  means  of 
one  of  these  machines,  the  electric  current  may  be 
made  to  work  machinery. 


Fig.  177. 


QUESTIONS. 

i.  Describe  the  first  method  of  developing  induced  cur- 
rents. 2.  The  second  method.  3.  The  third  method.  4.  By 
what  other  names  are  these  currents  known?  5.  Give  the  first 
illustration.  6.  The  second  illustration.  7.  The  third  illustra- 


NATURAL  PHILOSOPHY.  l8l 

tion.  8.  The  fourth  illustration.  9.  What  will  increase  the 
strength  of  the  current  in  the  last  two  cases?  10.  Describe 
the  induction  coil.  n.  Give  its  action.  12.  Describe  the  Bell 
telephone.  13.  Explain  its  action.  14.  Describe  the  carbon 
button.  15.  Explain  its  action.  16.  Describe  the  Edison  trans- 
mitter. 17.  Explain  the  action  of  the  Edison  telephone.  18. 
In  what  way  is  the  electric  current  developed  in  a  dynamo- 
electric  machine?  19.  Describe  the  armature  of  the  Edison 
machine.  20.  Describe  the  whole  machine.  21.  What  pro- 
duces the  current?  22.  What  strengthens  the  current?  23. 
What  is  the  effect  of  sending  an  outside  current  through  the 
armature  ? 


APPENDIX. 


THE  following  are  given  as  a  few  random  examples  of 
familiar  experiments  (with  apparatus  found  in  most  school  col- 
lections), that  may  be  used  as  additional 
illustrations,  or  for  purposes  of  review  and 
examination  (see  Preface) :  — 

Page  31.  —  The  fact  that  all  bodies  fall 
at  the  same  rate  in  a  vacuum  may  be 
mentioned,  and  shown  with  the  "guinea 
and  feather  tube"  (Figure  178). 

Page  78.  —  The  upward  pressure  of 
the  air  may  be  illustrated  by  the  "weight- 
lifter  "  (Figure  179);  and  the  fact  that  the 
air  presses  in  all  directions,  by  the  "  Mag- 
deburg hemispheres"  (Figures  180  and 
181),  which  take  their  name  from  Otto  von 
Guericke  of  Magdeburg,  by  whom  they 
were  invented. 

Page  79.  —  The  rise  of  liquids  in  ex- 
hausted vessels  is  strikingly  illustrated  by 
the  piece  of  apparatus  commonly  known 
as  the  "fountain  in  vacuo "  (Figure  182). 
The  bell-jar  is  first  exhausted,  and  the 
stop-cock  at  the  bottom  is  closed.  The 
end  of  the  tube  is  then  placed  under 
water,  and  the  stop-cock  opened,  when  the 
water  is  driven  up  into  the  bell-jar  in  a 
beautiful  fountain. 

Page  107.  —  The  fact  that  a  diminution 
of  pressure  lowers  the  boiling-point  may 
be  called  up,  and  Franklin's  experiment  (Figure  183)  per- 

183 


Fig.  178. 


1 84  APPENDIX. 

formed  to  illustrate  it.     The  water  in  the  flask  is  first  boiled, 


Fig.  179  Fig.  180 

to  expel  the  air;  and  the  flask,  after  being  removed  from  the 
source  of  heat,  is  tightly  corked.     It  is  then  arranged  as  in 

VM 


Fig.  181.  Fig.  182. 

the  figure,  and  cold  water  is  poured  over  it.     The  tension  of 


APPENDTX. 


I85 


the  steam  and  its  pressure  on  the  water  in  the  flask  are  thus 
reduced,  and  the, liquid  be- 
gins to  boil  again. 

Questions.  —  Will  water 
boil  at  the  same  tempera- 
ture on  the  top  of  a  moun- 
tain as  at  its  base  ?  Why  ? 
How  would  the  boiling-point 
be  affected  at  the  bottom 
of  a  deep  mine?  Why? 

Page  114.  —  The  effect 
of  evaporation  in  reducing 
the  temperature,  or  render- 
ing heat  latent,  may  be 
shown  by  the  experiment 
represented  in  Figure  184, 
which  requires  only  such 

apparatus  as  is  readily  extemporized.  A  little  water  is  put  in 
a  test-tube,  which  is  placed  in  a  wineglass  of  ether,  and  a  cur- 
rent of  air  blown 
through  the  ether 
by  means  of  the 
bellows.  The  wa- 
ter will  be  frozen 
in  a  very  short 
time. 

Page  145.— The 
use  of  the  pith 
ball  to  show  the 
presence  of  elec- 
tricity naturally 
suggests  its  use 
in  the  pith-ball 
electrometer  (Fig- 
ure 185),  which  the 
teacher  can  easily 
make,  if  it  is  not 


84. 


among  his  apparatus.     The  wooden  stem  C  is  mounted  in  a 


1 86 


APPENDIX. 


metal  socket,  by  which  it  can  be  attached  to  the  conductor 
whose  electrification  is  to  be  measured.  The  pith  ball,  fixed 
to  a  straw  stem  A,  is  hung  on  a  pivot  at  the  centre  of  the 
graduated  arc  B.  The  number  of  degrees  over  which  the 


Fig.   185. 


Fig.  186. 


Z 


straw  passes  affords  a  rough  measurement  of  the  strength  of 
the  electrification. 

Page  134.  —  In  connection  with  the  silent  discharge,  as 
illustrated  in  Figure  140,  the  "electric  wind,"  caused  by  the 
charging  of  the  molecules  of  air  in  front  of  the  point,  and 
their  consequent  repulsion,  may  be  shown 
(Figure  186).  The  "electric  mill"  (Figure 
187),  which  is  driven  by  the  reaction  of 
the  repelled  molecules  upon  the  point,  may 
also  be  introduced  here. 

The  Ley  den  Jar  (Figure  188)  serves  as 
a  striking  illustration  of  electrical  induction. 
When  the  inner  coating  is  charged  posi- 
tively from  the  prime  conductor  of  the 
electrical  machine,  the  outer  coating  be- 
comes charged  negatively  by  induction.  The  outer  coating, 
like  the  rubber  of  the  machine  (147),  must  be  connected  with 
the  earth. 

The  spark  discharge  is  well  illustrated  by  discharging  the 
jar  by  means  of  the  "discharging  rod"  (Figure  189).  The  jar 
may  be  discharged  gradually  and  silently  by  means  of  a  small 


Fig.  187. 


APPENDIX. 


I87 


metallic  ball  suspended  by  a  silk  thread,  so  as  to  swing  be- 
tween the  rod  from  which  it  is  hung  and  the  ball  of  the  jar 
(Figure  190).  The  rod  must  be  connected  by  a  strip  of1  tin-foil 


Fig.  188.  Fig.  i? 

at  its  base  (or  by  some  other  conductor)  with  the  outer  coating 
of  the  jar.  The  two  bells  shown  in  the  figure  are  not  neces- 
sary to  the  experiment,  but  add  to  its  effect  by  their  alternate 
ringing. 


/ 


Ffe  190. 

The  spark  discharge  is  also  prettily  illustrated  by  Ihe  "  span- 
gled pane"  (Figure  191).  A  long  strip  of  tin-foil  is  .pasted  in 
parallel  lines,  connected  at  alternate  ends,  between  a  knob  at 


i88 


APPENDIX. 


the  top  and  one  at  the  bottom  of  the  pane.     A  design  is  then 

traced  on  the  page  by  means  of  a 
sharp  point  which  cuts  through  the 
tin-foil.  A  discharge  of  electricity 
between  the  knobs  brings  out  the 
pattern  in  lines  of  light,  a  spark 
being  produced  wherever  the  foil 
has  been  cut.  The  rod  or  wire 
from  one  of  the  knobs  should  not 
quite  touch  the  discharging  rod  of 
the  electrical  machine.  An  inter- 
val of  half  an  inch  or  so  should  be 
left  for  sparks  to  pass. 

Many  forms  of  this  apparatus  are 
made ;  but  in  all,  the  light  is  pro- 
duced by  the  passage  of  electricity 
through  the  air  (151)  from  one 
piece  of  tin-foil  to  another.  Simple 

devices  of  the  kind  can  be  made  with  trifling  labor  and  expense 

by  the  teacher. 

NOTE.  —  Questions  suggested  by  the  experiments  (of  which  a  slight 
sample  is  given  on  p.  184  above)  will  readily  occur  to  the  teacher. 
Experiments  always  interest  and  amuse  the  pupil ;  but  if  he  is  not 
required  to  note  and  explain  what  is  done,  and  how  it  illustrates  phe- 
nomena not  mentioned  in  the  book,  the  exhibition  will  be  about  as 
profitable  to  him  as  a  display  of  fireworks,  or  a  dance  of  puppets  on  a 
hand-organ. 


INDEX. 


A. 

Action  and  reaction,  17. 
Affinity,  17. 
Air,  pressure  of,  78. 
Air-pump,  the,  69. 
Anode,  the,  161. 
Archimedes's  principle,  62. 
Artesian  wells,  72. 
Astronomy,  21. 
Atoms,  10,  21. 


Balloons,  66. 
Barometer,  the,  83. 
Beam  denned,  116. 
Bell's  telephone,  175. 
Body  defined,  i,  21. 
Boiling-point,  the,  107. 
Bunsen's  cell,  156. 


C. 

Camera  obscura,  the,  140. 

Capillarity,  75. 

Capstan,  the,  50. 

Carbon-button,  the,  177. 

Cathode,  the,  161. 

Centre  of  gravity,  37. 

Centrifugal  force,  25. 

Centripetal  force,  26. 

Chemistry,  21,  22. 

Clouds,  113. 

Cog-wheels,  50. 

Cohesion,  17. 

Coil,  the  induction,  175. 

Collision  of  elastic  bodies,  35. 

Color,  124. 

Colors,  prismatic,  122. 

Crab,  the,  54. 

Crane,  the,  54. 

Crystals,  73. 


D. 

Daniell's  cell,  157. 
Density,  4. 
Derrick,  the,  54. 
Discharge,  auroral,  155. 

electrical,  150. 

silent,  154. 

spark,  150. 
Dynamo-electrical  machines,  179. 


Echoes,  94. 

Edison's  dynamo-electrical  machine,  179 
phonograph,  96. 
telephone,  178. 
Elasticity,  18. 
Electrical  attraction,  144,  145. 

charge,  144. 

conductors,  144. 

current,  156. 

discharge,  150. 

excitation,  145. 

illumination,  158,  159. 

induction,  146. 

insulators,  145. 

lamp,  160. 

machines,  146. 

repulsion,  144,  145. 
Electricity,  frictional,  144. 

voltaic,  156. 
Electrodes,  160. 
Electrolysis,  160. 
Electro-magnetism,  164. 
Electro-metallurgy,  161. 
Electrometer,  pith-ball,  185. 
Electrophorus,  the,  149. 
Electroplating,  162. 
Electrotyping,  161. 
Energy,43. 
Equilibrium,  39. 
Ether,  the,  n. 
Evaporation,  107. 
Eye,  the  human,  142. 

189 


190 


INDEX. 


F. 

Falling  bodies,  31. 
Floating  bodies,  66. 
Fluids,  60. 

Foot-pound  defined,  45. 
Foot-poundal  defined,  45. 
Force  defined,  16. 
Force-pump,  the,  80. 
Forces,  the  three  great,  16. 
measurement  of,  19. 
Freezing-point,  the,  107. 
Fusion,  107. 


G. 

Galvanometers,  166. 
Gases,  59,  68. 

cohesion  in,  59. 

diffusion  of,  68. 

expansion  of,  68,  105. 
Gold-leaf,  75. 
Gravity,  16. 

centre  of,  37. 

H. 

Heat,  absorption  of,  100. 

conduction  of,  101. 

consumed  in  evaporation,  114. 
expansion,  112. 
liquefaction,  113. 

convection  of,  102. 

expansion  by,  104. 

latent,  112. 

nature  of,  99. 

radiation  of,  100. 

sensible,  112. 

specific,  109. 

unit  of,  109. 

Holtz  electrical  machine,  149. 
Hydraulic  press,  the,  61. 
Hydrometers,  67. 


Impulse  defined,  28. 

Inclined  plane,  the,  51. 

Induction  coils,  175. 

Inertia,  24. 

Images,  formed  by  lenses,  138,  140. 

from  small  apertures,  117. 

in  concave  mirrors,  130. 

in  convex  mirrors,  132. 

in  plane  mirrors,  127. 


Lenses,  forms  of,  133. 

images  formed  by,  138,  140. 
Lever,  the,  48. 
Leyden  jar,  the,  186. 


Light,  absorption  of,  124. 
diffusion  of,  123. 
dispersion  of,  121. 
nature  of,  116. 
radiation  of,  116. 
reflection  of,  119. 
refraction  of,  120,  134,  139 
velocity  of,  9. 
Lightning,  153. 
Lightning-rods,  154. 
Liquids,  cohesion  in,  59. 

compressibility  of,  70. 
evaporation  of,  107. 
expansion  of,  105. 
pressure  of,  71. 
Luminous  bodies,  116. 


M. 

Machines,  47. 

law  of,  56. 
uses  of,  52. 
work  done  by,  56. 
Magnetic  needles,  164. 
Magneto-electricity,  174. 
Magnets,  164. 
Mass  defined,  4. 
Material  universe,  the,  6. 
Matter,  compressibility  of,  12. 

defined, i. 

divisibility  of,  13. 

impenetrability  of,  13. 

indestructibility  of,  14 

porosity  of,  12. 

three  states  of,  59. 
Mechanical  powers,  47. 
Mechanics,  21. 
Melting-point,  the,  107. 
Mirrors,  concave,  125,  128. 
convex,  125. 

plane,  125,  127. 
Molecules,  10,  21. 
Momentum,  29. 
Motion,  first  law  of,  23. 

molar,  9. 

molecular,  u. 

parallelogram  of,  30, 

reflected,  36'. 

second  law  of,  28. 

third  law  of,  34. 


N. 

Natural  philosophy,  22. 

Noise,  88. 

Northern  Lights,  the,  155. 


O. 

Octave  defined,  88. 
Opaque  bodies,  116. 


INDEX. 


IQI 


P. 

Pascal's  law,  60.  * 

Phenomenon  defined,  20. 
Phonograph,  the,  96. 
Physical  sciences,  21. 
Physics,  21,  22. 
Planets,  6. 
Pores,  12. 

Position  of  advantage,  42. 
Poundal  denned,  20. 
Prismatic  colors,  122. 
Properties,  chemical,  21. 
physical,  21. 
Pulley,  the,  50. 
Pumps,  80. 


R. 

Rain,  113. 

Ray  denned,  116. 

Reaction,  17,  34- 

Reeds,  90. 

Reflection,  law  of,  36,  94,  119. 

Relay,  the,  170. 

Resistance,  24. 

Rest  defined,  10. 

Rising  bodies,  32. 


Screw,  the,  52. 
Senses,  the,  i. 
Shadows,  118. 
Siphon,  the,,  81. 
Snow-crystals,  74. 
Solar  system,  7. 
Solids,  cohesion  in,  59. 

expansion  of,  105. 

properties  of,  74. 
Sound,  intensity  of,  88. 

origin  of,  86. 

pitch  of,  88. 

propagation  of,  91. 

quality  of,  87. 

reflection  of,  94. 

velocity  of,  92,  93. 

waves,  91,  92. 
Specific  gravity,  63,  65. 
Spectrum,  the,  122. 
Springs,  72,  83. 
Strain  defined,  18. 
Stress  defined,  17. 
Stringed  instruments,  89. 
Substance  defined,  i . 
Suction-pump,  the,  80. 


T. 

Tantalus's  cup,  82. 
Telegraph  key,  the,  167. 

relay,  170. 

sounder,  169. 
Telephone,  Bell's,  175. 

Edison's,  178. 
Temperature  defined,  108. 
Thermometer,  the,  109. 
Thunder,  153.  0 

Torricelli's  experiment,  79. 
Transparent  bodies,  116. 


U. 

Unison  defined,  88. 
Units,  material,  21. 

of  length,  2. 

of  mass,  4. 

of  surface,  3. 

of  volume,  3. 
Universe,  the  material,  6. 
the  stellar,  9. 


V. 

Vapors,  107. 
Velocity,  23. 

Vibrations,  fundamental,  87. 
harmonic,  87. 
heat,  99. 
light,  116. 
molar,  86. 
sympathetic,  94. 
Visual  angle,  the,  125. 
Voltaic  arc,  the,  158. 

battery,  the,  157. 
cell,  the,  156. 

Bunsen's,  156. 
Daniell's,  157. 
zinc  and  copper,  156. 
electricity,  156. 


W. 

Water,  expansion  of,  106. 
Wedge,  the,  51. 
Wheel  and  axle,  the,  49. 
Wheel-work,  54. 
Wind  instruments,  89. 
Windlass,  the,  50. 
Work  defined,  42. 


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